What is Double Brace initialization in Java?

Question

What is Double Brace initialization syntax ({{ ... }}) in Java?

Answer 1

To avoid all negative effects of double brace initialization, such as:

1. Broken "equals" compatibility.
2. No checks performed, when use direct assignments.
3. Possible memory leaks.

do next things:

1. Make separate "Builder" class especially for double brace initialization.
2. Declare fields with default values.
3. Put object creation method in that class.

Example:

public class MyClass {
    public static class Builder {
        public int    first  = -1        ;
        public double second = Double.NaN;
        public String third  = null      ;

        public MyClass create() {
            return new MyClass(first, second, third);
        }
    }

    protected final int    first ;
    protected final double second;
    protected final String third ;

    protected MyClass(
        int    first ,
        double second,
        String third
    ) {
        this.first = first ;
        this.second= second;
        this.third = third ;
    }

    public int    first () { return first ; }
    public double second() { return second; }
    public String third () { return third ; }
}

Usage:

MyClass my = new MyClass.Builder(){{ first = 1; third = "3"; }}.create();

Advantages:

1. Simply to use.
2. Do not breaks "equals" compatibility.
3. You can perform checks in creation method.
4. No memory leaks.

Disadvantages:

• None.

And, as a result, we have simplest java builder pattern ever.

See all samples at github: java-sf-builder-simple-example