OCA Java SE 8 Core Java APIs - Java Class Equals

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Understanding Equality

== operator compares numbers and that object references refer to the same object.

public class Main{
   public static void main(String[] argv){
        StringBuilder one = new StringBuilder(); 
        StringBuilder two = new StringBuilder(); 
        StringBuilder three = one.append("a"); 
        System.out.println(one == two); // false 
        System.out.println(one == three); // true 
   }
}

one == two is false since they are pointing to different object instance. one == three is true since one.append("a") return one as the result.

The code above generates the following result.

Example

In the following example x == y returns true because the JVM reuses String literals:

Strings are immutable and literals are pooled.

The JVM created only one literal in memory. x and y both point to the same location in memory.

public class Main{
   public static void main(String[] argv){
        String x = "Hello World"; 
        String y = "Hello World"; 
        System.out.println(x == y);    // true 
   }
}

The code above generates the following result.

Example 2

The following code shows how the trim() method would create new String object.

In the code we don't have two of the same String literal.

Although x and z have the same string value, z is computed at runtime.

Since the value returned from trim() isn't the same at compile-time, a new String object is created.

public class Main{
   public static void main(String[] argv){

        String x = "Hello World"; 
        String z = " Hello World".trim(); 
        System.out.println(x == z); // false 
   }
}

The code above generates the following result.

Example 3

We can create a new String without using the pooled value by using new operator:

public class Main{
   public static void main(String[] argv){
        String x = new String("Hello World"); 
        String y = "Hello World"; 
        System.out.println(x == y); // false 
   }
}

The code above generates the following result.

Example 4

The equals() method does the comparison for the content of two string values.

public class Main{
   public static void main(String[] argv){
        String x = "Hello World"; 
        String z = " Hello World".trim(); 
        System.out.println(x.equals(z)); // true 
   }
}

The code above generates the following result.

Example 5

In the following code, the first two statements check object reference equality.

The t1.equals(t2) prints false since Main does not implement equals().

public class Main { 
   String name; /* w ww.  j a v  a 2s. co  m*/
   public static void main(String[] args) { 
      Main t1 = new Main(); 
      Main t2 = new Main(); 
      Main t3 = t1; 
      System.out.println(t1 == t1); // true
      System.out.println(t1 == t2); // false
      System.out.println(t1.equals(t2)); // false
   }
}

The code above generates the following result.

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