Details of new jdk13 features

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In Java 13, the Test.java：Switch Expressions extends the previousJava 12 Switch Expressions by adding a new yieldkeyword to return a value from switch expression.

P.S ：Switch expressions are a preview feature and are disabled by default.

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1. No more value breaks!

1.1 The below Java 12 value breaks syntax is no longer compiled in Java 13, uses yieldinstead.

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	// value breaks are superseded by Java 13 'yield' statements. 
	private static int Test(String demo) {
        int result = switch (demo) {
            case "a":
            case "b":
                break 1;
            case "c":
                break 2;
            case "d":
            case "e":
            case "f":
                break 3;
            default:
                break -1;
        };
        return result;
    }
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1.2 In Java 13, uses yield to return a value from switch

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private static int Test(String demo) {
        int result = switch (demo) {
            case "a", "b":
                yield 1;
            case "c":
                yield 2;
            case "d", "e", "f":
                yield 3;
            default:
                yield -1;
        };
        return result;
    }

1.3 Return values vialabel rules or arrows like lambod expression from a switch expression (Java 12) is still supported in Java 13.

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private static int Test(String demo) {
       int result = switch (demo) {
           case "a", "b" -> 1;
           case "c" -> 2;
           case "d", "e", "f" -> 3;
           default -> -1;
       };
       return result;
   }

2. Java 13 Switch Expressions

A full switch expression example in Java 13.

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package com.chanzin.java13;

public class Test {

    public static void main(String[] args) {

        System.out.println(getValueViaYield("a"));
        System.out.println(getValueViaYield("c"));
        System.out.println(getValueViaYield("e"));
        System.out.println(getValueViaYield("z"));

    }

    // Traditional switch
    private static int getValueBefore12(String demo) {
        int result;
        switch (demo) {
            case "a":
            case "b":
                result = 1;
                break;
            case "c":
                result = 2;
                break;
            case "d":
            case "e":
            case "f":
                result = 3;
                break;
            default:
                result = -1;
        }
        ;
        return result;
    }

    
    private static int getValueMultipleLabels(String demo) {
        int result;
        switch (demo) {
            case "a", "b":
                result = 1;
                break;
            case "c":
                result = 2;
                break;
            case "d", "e", "f":
                result = 3;
                break;
            default:
                result = -1;
        }
        ;
        return result;
    }

    // Java 13, value breaks are superseded by 'yield' statements
    // Java 12, switch expression returning value via break
    /*private static int getValueViaBreak(String mode) {
        int result = switch (mode) {
            case "a":
            case "b":
                break 1;
            case "c":
                break 2;
            case "d":
            case "e":
            case "f":
                break 3;
            default:
                break -1;
        };
        return result;
    }*/

    // Java 12, switch expression returning value via label rules (arrow)
    private static int getValueViaLambod(String demo) {
        int result = switch (demo) {
            case "a", "b" -> 1;
            case "c" -> 2;
            case "d", "e", "f" -> 3;
            default -> -1;
        };
        return result;
    }

    // Java 13, switch expression returning value via yield
    private static int getValueViaYield(String demo) {
        int result = switch (demo) {
            case "a", "b":
                yield 1;
            case "c":
                yield 2;
            case "d", "e", "f":
                yield 3;
            default:
                yield -1;
        };
        return result;
    }

}
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3. Enable preview features

3.1 A normal javac compile will prompts the following errors:

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3.2 We need to enable the preview feature manually:

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javac --enable-preview --release 13 Test.java


java --enable-preview Test


javap -c Test

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