ReferenceSamples.java

public class ReferenceSamples {
    public static void main(String[] args) {
        references();
        equality();
    }

    private static void equality() {
        Integer i1 = 1;
        int i2 = i1;
        assert i1 == i2 && i1.equals(i2);
        //assert i2.equals(i1); "error: int cannot be dereferenced"

        float f1 = 1.0f;
        Float f2 = f1;
        assert i1 == f1 && !i1.equals(f1);
        // assert i1 == f2; "error: incomparable types: Integer and Float"
        assert !i1.equals(f2);
        assert i1.intValue() == f2 && i1 == f2.intValue();
    }

    private static void references() {
        TestClass t = new TestClass();
 
        String[] a = {"one", "two", "three"};
        t.arrays(a);
        assert a[0] == "aaa";
        assert a[1] == "aaa";
        assert a[2] == "aaa";

        // reference final method are mutable!
        Holder h1 = new Holder(42);
        t.add(h1, 10);
        assert h1.getI() == 52;
                
        // strings are immutable, any value change creates a new reference 
        String s1 = new String("1");
        String s2 = s1;
        assert s1 == s2 && s1.equals(s2);
        
        s1 = new String("1");
        assert s1 != s2 && s1.equals(s2);
 
        // stringbuilder is immutable too
        StringBuilder sb1 = new StringBuilder("Hello");
        StringBuilder sb2 = sb1;
        assert sb1 == sb2 && sb1.toString().equals(sb2.toString()); //equals is not overriden
 
        sb2.append(" world"); // yep, that's silly
        assert sb1 == sb2 && sb1.toString().equals(sb2.toString());
 
        // different with mutable objects
        Holder h2 = new Holder(1);
        Holder h3 = h2;
        assert h2 == h3 && h2.getI() == h3.getI();
        
        h3.setI(5);
        assert h2 == h3 && h2.getI() == h3.getI();
 
        h2 = new Holder(5);
        assert h2 != h3 && h2.getI() == h3.getI();
    }
}
 
class TestClass {
    void add(final Holder h, int i) {
        int n = h.getI();
        h.setI(n + i);
        // compile error > h = new Holder(999);
    }

    void arrays(String[] array) {
        int i = 0;
        for (String s : array) {
            array[i++] = "aaa";
        }
    }
}
 
class Holder {
    private int i;
 
    public Holder(int i) {
        this.i = i;
    }
 
    public void setI(int i) {this.i = i;}
    public int getI() {return i;}
 
    public String toString() { return "Hold: " + i; }
}