equals() Method

• Non-final method of java.lang.Object class.

  • public boolean equals(Object other);

• Definition of Object.equals():

  

  public boolean equals(Object obj) {
      return (this == obj);
  }
  

  

• To compare the object contents/state, the programmer should override the equals() method.

• Properties of equals() method:

  • Reflexive: for any non-null reference value x, x.equals(x) should return true.
  • Symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
  • Transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
  • Consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
  • For any non-null reference value x, x.equals(null) should return false.

Example:

class Employee {
	// ...
	@Override
	public boolean equals(Object obj) {
		if(obj == null)
			return false;
			
		if(this == obj)
			return true;
			
		if(! (obj instanceof Employee))
			return false;
			
		Employee other = (Employee) obj;
		if(this.id == other.id)
			return true;
		return false;
	}
}

Garbage Collection

• Garbage collection is automatic memory management by JVM.

• If a Java object is unreachable (i.e. not accessible through any reference), then it is automatically released by the garbage collector.

• An object become eligible for GC in one of the following cases:

  • Nullifying a reference:

    MyClass obj = new MyClass();
    obj = null;
    

  • Reassigning a reference:

    MyClass obj = new MyClass();
    obj = new MyClass();
    

  • Object created locally in a method:

    void method() {
        MyClass obj = new MyClass();
        // ...
    }
    

• GC is a background thread in JVM that runs periodically and reclaim memory of unreferenced objects.

• Before object is destroyed, its finalize() method is invoked (if present).

• One should override this method if object holds any resource to be released explicitly e.g. file close, database connection, etc.

Example: