Operators and Assignments - Boolean equals()

• defined in java.lang.Object therefore inherited by all classes
• returns true if and only if the two variables being compared hold a reference to the same object

To check if objects are of the same class use the Comparison operator: instanceof

    Class_A a = new Class_A();
    Class_B b = new Class_B();
    Class_C c = new Class_A();
    Class_B d = b;
    Class_A e = null;
    
    a.equals(b)             // false (different obj refs)
    a.equals(c)             // false (different obj refs)
    b.equals(d)             // true  (same object refs)    
    a.equals(e)             // false (always returned when 
                            // compared to a null)

• java.lang.String overrides the java.lang.Object equals() method to return true if and only if the objects being compared contain the same sequence of characters.

    String s0 = "Hello";
    String s1 = new String("Hello");    // force new string object
    String s2 = s0;
    
    s0.equals(s1)           // true (diff objects, same chars)
    s0.equals(s2)           // true (same chars, coincidence
                            //       they are same objects)

• java.lang.Boolean overrides the java.lang.Object equals() method, returning true if and only if the Boolean objects represent the same boolean value

    Boolean b0 = new Boolean(true);
    Boolean b1 = new Boolean(false);
    Boolean b2 = new Boolean(true);
    Boolean b3 = b1;
    
    b0.equals(b1)           // false (different boolean values)
    b0.equals(b2)           // true  (same boolean values)
    b1.equals(b3)           // true  (same boolean values)

    Boolean b3 = new Boolean();
    boolean b4 = true;
    
    b3 = b4;      // compile-error: incompatible types
    b3 = true;    // compile-error: incompatible types

Example Code

• TestBooleanEquals.java

Tips

• all the primitive type wrapper classes override the Object.equals() method to compare the value of the objects; the default Object.equals() checks if the variables reference the same object