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@ -1,6 +1,6 @@
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# Java
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```java
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```java linenums="1"
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//single line comment
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/* multi line comment */
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/** javaDoc docstring */
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@ -19,7 +19,7 @@ Package definition: `package <package_location>;`
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### Main Method (entry point of algorithm)
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```java
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```java linenums="1"
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public static void main (String[] args) {
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//code here
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}
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@ -31,21 +31,21 @@ public static void main (String[] args) {
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### Constant Definition (outside of main method/function)
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```java
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```java linenums="1"
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public static final Type CONSTANT_NAME = value;
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public static final double PI = 3.14159; //example
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```
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### Constant Definition (inside main method/function)
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```java
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```java linenums="1"
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final Type CONSTANT_NAME = value;
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final double PI = 3.14159; //example
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```
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### Screen Output
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```java
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```java linenums="1"
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System.out.println(output_1 + _ + output_n); //newline at every invocation
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System.out.print(output_1 + _ + output_n);
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```
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@ -54,7 +54,7 @@ System.out.print(output_1 + _ + output_n);
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[String.format() Examples](https://dzone.com/articles/java-string-format-examples)
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```java
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```java linenums="1"
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System.out.printf("string %..", variable);
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System.out.println(String.format(format, args));
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```
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@ -65,7 +65,7 @@ Methods inherited from C. The value pf the variable substitutes %.
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`NumberFormat` class is used to format a number output.
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```java
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```java linenums="1"
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Locale locale = new Locale("language", "country"); // as defined by IETF lang tag, RCF 5646, RCF 4647
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NumberFormat fmt = NumberFormat.getCurrencyInstance(locale); // format a number as a currency based on a Locale
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fmt.format(number); // apply format to a number, returns a String
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@ -73,7 +73,7 @@ fmt.format(number); // apply format to a number, returns a String
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## Keyboard Input
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```java
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```java linenums="1"
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import java.util.Scanner; //package import
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Scanner scanner = new Scanner(System.in); //Scanner obj init
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scanner.useDelimiter("delimitatore"); //delimiter setting
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@ -92,7 +92,7 @@ Thus when switching to a different input method is necessary to call `nextLine()
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### Primitive Types
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```java
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```java linenums="1"
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TYPE WRAPPER SIZE MIN_VALUE MAX_VALUE
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int Integer -2147483648 2147483647
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byte Byte 8 bit -128 127
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@ -115,7 +115,7 @@ For high precision calcs is best to use `BigDecimal`.
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### Type Conversion (casting) & Type checking
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```java
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```java linenums="1"
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Type variable = (Type) <expression>; // convert to other Type
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var instanceof Type; // true if var is an instance of Type
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```
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@ -125,7 +125,7 @@ var instanceof Type; // true if var is an instance of Type
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Every primitive type has a corresponding wrapper class.
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Wrapper classes permits the creation of an object with the same type of a primitive type but with added methods and constants.
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```java
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```java linenums="1"
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WrapperClass objectName = new WrapperClass(primitiveValue); //declaration
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WrapperClass objectName = primitiveValue; //shortened declaration
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@ -143,7 +143,7 @@ WrapperClass.toString(primitive); // converts the wrapper class value to a stri
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### String & Char
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```java
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```java linenums="1"
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String string = "text"; //strings always in double quotes
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char character = 'C'; //chars always in single quotes
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```
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@ -172,7 +172,7 @@ String are immutable. Concatenation creates a new string.
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### String Conversion to Number
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```java
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```java linenums="1"
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double d = Double.parseDouble(string);
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float f = Float.parseFloat(string);
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int i = integer.parseInt(string);
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@ -180,7 +180,7 @@ int i = integer.parseInt(string);
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### String Class Methods
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```java
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```java linenums="1"
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string.equals(otherString); // returns TRUE if the strings are equal
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string.equalsIgnoreCase(otherString); // returns TRUE if the strings are equals ignoring the case
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string.charAt(index); // returns the character at position INDEX
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@ -211,7 +211,7 @@ To compare in alphabetical order both strings must have the same case.
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### Mathematical Operations
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```java
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```java linenums="1"
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Math.PI // value of pi
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Math.E // value of e
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@ -317,7 +317,7 @@ Full evaluation can be forced using `&` and `|`.
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### `If Else`
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```java
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```java linenums="1"
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if (condition) {
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//code here
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} else {
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@ -327,7 +327,7 @@ if (condition) {
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### `If, Else If, Else`
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```java
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```java linenums="1"
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if (condition) {
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//code here
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} else if (condition) {
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@ -346,7 +346,7 @@ if condition is `true` executes instruction1 otherwise executes instruction2.
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### `Switch`
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```java
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```java linenums="1"
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switch (matchExpression) {
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case matchingPattern:
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//code here
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@ -366,7 +366,7 @@ Omitting the `break` keyword causes multiple branches to execute the same code.
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### `While` Loop
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```java
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```java linenums="1"
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while (condition) {
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//code here
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}
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@ -374,7 +374,7 @@ while (condition) {
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### `Do While` Loop
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```java
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```java linenums="1"
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do {
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//code here
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} while (espressione_booleana);
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@ -384,7 +384,7 @@ Loop body executed *at least* one time
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### `For` Loop
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```java
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```java linenums="1"
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for (initializer; condition; iterator) {
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//code here
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}
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@ -392,7 +392,7 @@ for (initializer; condition; iterator) {
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### `Foreach` Loop
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```java
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```java linenums="1"
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for (Type variable : iterable){
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//code here
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}
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@ -400,7 +400,7 @@ for (Type variable : iterable){
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### Multiple iterators in `For` Loop
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```java
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```java linenums="1"
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for (initializer1, initializer2; condition; iterator1, iterator2) {
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//code here
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}
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@ -417,7 +417,7 @@ The iterator declared in the for is a local variable and can be used only in the
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If the assertion check is enabled (`java -ebableassetrions programName`) the execution of the algorithm is terminated if an assertion fails.
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Assertions can be used to check if a variable has a wanted value in a precise point in the code (Sanity Check).
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```java
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```java linenums="1"
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assert <booleanExpression>;
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```
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@ -427,7 +427,7 @@ Static methods are not bound to an *instance* of a class but they act on the cla
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### Static Void Method Definition
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```java
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```java linenums="1"
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static void methodName (parameters) {
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//code here
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}
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@ -435,7 +435,7 @@ static void methodName (parameters) {
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### Static Method Definition
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```java
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```java linenums="1"
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static tipo_metodo methodName (parameters) {
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//code here
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return <espressione>; //returned type MUST match method type
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@ -444,7 +444,7 @@ static tipo_metodo methodName (parameters) {
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### Static Method Invocation
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```java
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```java linenums="1"
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methodName(arguments);
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ClassName.methodName(arguments); //if method is used outside its class
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```
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@ -453,7 +453,7 @@ ClassName.methodName(arguments); //if method is used outside its class
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## Array Declaration
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```java
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```java linenums="1"
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Type[] arrayName = new Type[dimension];
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Type arrayName[] = new Type[dimension];
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@ -463,7 +463,7 @@ arrayName.length //length of the array
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Its possible to break the declaration in two lines
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```java
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```java linenums="1"
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Type[] arrayName;
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arrayType = new Type[dimension];
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```
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@ -475,7 +475,7 @@ Array dimension is determined by the number of values.
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### Arrays as method parameters
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```java
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```java linenums="1"
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static Type methodName (Type[] arrayName) {
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//code here
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}
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@ -492,7 +492,7 @@ Array contents must be confronted by looping through the array.
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### Methods returning Arrays
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```java
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```java linenums="1"
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static Type[] methodName (parameters) {
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Type[] arrayName = new Type[dimension]; //array declaration
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//array valorization
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@ -502,7 +502,7 @@ static Type[] methodName (parameters) {
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## Variable numbers of parameters in a method
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```java
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```java linenums="1"
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static Type methodName (parameters, tipo[] ArrayName) {
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//code here
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}
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@ -512,14 +512,14 @@ It's not necessary to specify a dimension of the array, it's determined by Java
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### Multi-Dimensional Arrays
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```java
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```java linenums="1"
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Type[]...[] arrayName = new Type[dimension1]...[dimensionN];
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Type arrayName[]...[] = new Type[dimension1]...[dimensionN];
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```
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### Multi-Dimensional Arrays as parameters
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```java
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```java linenums="1"
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static Type methodName (Type[]...[] ArrayName) {
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//code here
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}
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@ -527,7 +527,7 @@ static Type methodName (Type[]...[] ArrayName) {
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### Methods returning multi-dimensional arrays
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```java
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```java linenums="1"
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static Type[]...[] methodName (parameters) {
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Type[]...[] array = new Type[dimension1]...[dimensionN];
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//array valorization
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@ -537,14 +537,14 @@ static Type[]...[] methodName (parameters) {
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### Array Length of multi-dimensional arrays
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```java
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```java linenums="1"
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array.length //row length
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array[rowIndex].length //column length
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```
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### Irregular Table Visualization
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```java
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```java linenums="1"
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static void viewTable (Type[][] matrix){
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for (int row = 0; row < matrix.length; row++){ //run through the rows
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for (int column = 0; column < arrayName[row].length; column++){ //run through the columns
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@ -571,7 +571,7 @@ Unchecked exceptions usually mean that there is an error in the logic of the pro
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This construct permits to monitor what happens in a block of code and to specify what to do in case of errors (*exceptions*).
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```java
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```java linenums="1"
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try {
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//monitored code
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} catch (SpecificException e) {
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@ -590,7 +590,7 @@ A `try-catch` construct can handle multiple exceptions at once. Every `catch` i
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### Try with Resources
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```java
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```java linenums="1"
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try (
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//resource definition
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){
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@ -607,7 +607,7 @@ try (
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The `throw` keyword is used to generate a custom exception in a point of the code.
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`throw` is used together with an exception type.
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```java
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```java linenums="1"
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Type methodName(parameters) {
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if (condition) {
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throw new Exception("error message");
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@ -618,7 +618,7 @@ Type methodName(parameters) {
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The `throws` keyword is used to indicate what exception Type may be thrown by a method.
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`throws` is used together with a exception class. It's used to send the exception to the method caller.
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```java
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```java linenums="1"
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Type methodName(parameters) throws ExceptionClass {
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if (condition) {
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throw new SpecificException("error message");
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@ -630,7 +630,7 @@ Type methodName(parameters) throws ExceptionClass {
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A user-defined exception has to inherit from `Exception` or one of it's descendants.
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```java
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```java linenums="1"
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public class CustomException extends Exception {
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public CustomException(){
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@ -659,7 +659,7 @@ If not specified variables, methods and classes are *only* accessible from the s
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### Instance Method Definition
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```java
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```java linenums="1"
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Type methodName (parameters) {
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//code here
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}
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@ -667,7 +667,7 @@ Type methodName (parameters) {
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### `Void` Instance Method
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```java
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```java linenums="1"
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void methodName (parameters) {
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//code here
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}
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@ -675,7 +675,7 @@ void methodName (parameters) {
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### Class Definition
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```java
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```java linenums="1"
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public class ClassName {
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//instance variables declaration
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@ -711,7 +711,7 @@ Constructors are special methods that are invoked with the `new` operator when a
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Constructors assign the starting values of the object attributes.
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If a constructor id defined Java doesn't create the default constructor.
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```java
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```java linenums="1"
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class ClassName (){
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//attributes declaration (aka instance variables)
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@ -736,7 +736,7 @@ Static variable do not belong to the class objects.
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### Getters & Setter Methods
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```java
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```java linenums="1"
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public void setAttribute(Type attribute){
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this.attribute = attribute;
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}
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@ -750,7 +750,7 @@ public Type getAttribute(){
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Automatically returns a string if the object is directly called in a print method.
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|
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```java
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```java linenums="1"
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@Override
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Public String toString(){
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return "string-representation-of-object"
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|
|
@ -777,7 +777,7 @@ A child class can inherit from *only* one parent class.
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Child class **must** implement a constructor that instantiates the parent (super) class.
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`super()` instantiates the superclass of the child class.
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|
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```java
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```java linenums="1"
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class ChildClass extends ParentClass{
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public ChildClass(parentParameters, childParameters){
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|
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@ -800,7 +800,7 @@ An overridden method can change the access modifier as long as the new modifier
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A `ParentClass` type can contain a `ChildClass` object. This is useful for using collections and arrays of objects.
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```java
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```java linenums="1"
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ParentClass objectName = ChildClass(); // upcast
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(ChildClass)ParentClassObject; // downcast
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```
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|
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@ -814,7 +814,7 @@ Abstract classes are marked by the `abstract` keyword.
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An abstract method is a method without implementation.
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The methods **must** be `public` and marked with the `abstract` keyword.
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|
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```java
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```java linenums="1"
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//abstract class
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abstract class className{
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//attributes here
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|
|
@ -838,7 +838,7 @@ Interfaces' attributes are always `public static final`, no need for the keyword
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|
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A class can implement *more than one* Interface.
|
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|
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```java
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```java linenums="1"
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public interface InterfaceName{
|
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//attributes here
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||||
|
||||
|
|
@ -869,7 +869,7 @@ Enums are used to restrict the type of data to a set of the possible constant va
|
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Enums are classes which constructor is private by default.
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It's still possible to create a custom constructor to add values.
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|
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```java
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```java linenums="1"
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enum enumName {
|
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value1,
|
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value2,
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|
|
@ -904,7 +904,7 @@ enumName variable; //creation of a variable of type enumName
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|
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*Anonymous classes* make the code more concise. They enable to declare and instantiate a class at the same time. They are like local classes except that they do not have a name. Useful if is needed a local class that is used once.
|
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|
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```java
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```java linenums="1"
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AnonymousClass objectName = new AnonymousClass(Type parameter, ...) {
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|
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// attributes
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||||
|
|
@ -915,7 +915,7 @@ AnonymousClass objectName = new AnonymousClass(Type parameter, ...) {
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|
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### Cloning
|
||||
|
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```java
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```java linenums="1"
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class ClassName implements Clonable {
|
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|
||||
}
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|
|
@ -923,7 +923,7 @@ class ClassName implements Clonable {
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|
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## Generics
|
||||
|
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```java
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||||
```java linenums="1"
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// WARNING: T is not instantiable, new t(), new t[] are INVALID
|
||||
public class GenericClass<T> {
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private T generic;
|
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|
|
@ -949,7 +949,7 @@ GenericClass<Type>[] obj = new GenericClass<>[]; // invalid
|
|||
|
||||
### Multiple Generics
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
public class GenericClass<T1, T2, ...> { } // number of generic types is not limited
|
||||
```
|
||||
|
||||
|
|
@ -957,7 +957,7 @@ public class GenericClass<T1, T2, ...> { } // number of generic types is not li
|
|||
|
||||
Specify an interface or class that the generic type must implement/inherit.
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
public class GenericClass<T extends Interface1 & Interface2> { }
|
||||
public class GenericClass<T1 extends Interface1 & Interface2, T2 extends Class1> { }
|
||||
public class GenericClass<T extends Class1> { }
|
||||
|
|
@ -965,7 +965,7 @@ public class GenericClass<T extends Class1> { }
|
|||
|
||||
### Generic Methods
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
public class ClassName{
|
||||
|
||||
public <T> methodName() {
|
||||
|
|
@ -1001,7 +1001,7 @@ obj.<Type>methodName(); // generic method call
|
|||
|
||||
#### Writing on a file
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
// opening/creating the file for writing
|
||||
PrintWriter outStream = null; // output stream creation
|
||||
try{
|
||||
|
|
@ -1022,7 +1022,7 @@ outStream.close() // close stream and write buffer contents.
|
|||
|
||||
#### Reading from a file
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
Filereader filereader = new Filereader("filename"); //open the file
|
||||
Scanner scanner = new Scanner(filereader); //scanner for the file
|
||||
|
||||
|
|
@ -1058,7 +1058,7 @@ public void close() throws IOException // closes the stream
|
|||
|
||||
The File class is an abstraction of the file and it's path. The abstraction is independent from the OS.
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
File("path/to/file") // UNIX like path
|
||||
File("path\\to\\file") // Windows path
|
||||
|
||||
|
|
@ -1075,7 +1075,7 @@ file.length() // file length in bytes
|
|||
|
||||
#### Writing to a binary file
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
ObjectOutputStream outStream;
|
||||
try {
|
||||
outStream = new ObjectOutputStream(new FileOutputSteam(filename));
|
||||
|
|
@ -1104,7 +1104,7 @@ public void close()
|
|||
|
||||
#### Reading from a binary file
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
ObjectInputStream inStream;
|
||||
try {
|
||||
inStream = new ObjectInputStream(new FileinputSteam(filename));
|
||||
|
|
@ -1148,7 +1148,7 @@ Needed for a class to be *serializable*:
|
|||
|
||||
An array is serializable if it's base type is a serializable object.
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
SerializableObject[] array = (SerializableObject[])inStream.readObject(); // read returns Object, cast needed
|
||||
```
|
||||
|
||||
|
|
@ -1160,7 +1160,7 @@ Functional interfaces provide target types for *lambda expressions*.
|
|||
|
||||
General purpose `@functionalInterfaces`:
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
// takes input, performs actions, return boolean
|
||||
public interface Predicate<T> {
|
||||
boolean test(T t);
|
||||
|
|
@ -1215,7 +1215,7 @@ The features of Java stream are:
|
|||
|
||||
Usable only by a `@FunctionalInterface`'s method or a method of a *stream*.
|
||||
|
||||
```java
|
||||
```java linenums="1"
|
||||
lambda operator -> body;
|
||||
|
||||
//zero parameter
|
||||
|
|
|
|||
Loading…
Add table
Add a link
Reference in a new issue