Reference Comparison

The == operator compares the values in variables.

With primitives:

int x = 17;
int y = 17;
int z = 42;

System.out.println(x == y); // true
System.out.println(x == z); // false

With references:

String c = new String("cat"); // A new object on the heap
String d = new String("cat"); // Another new String object
System.out.println(c == d);   // false
                              // The references to the two Strings are different

We've learned that to compare the actual contents of two String objects, we use equals().

System.out.println(c.equals(d)); // true

The String class provides the equals() method to allow comparison of the contents of two Strings instead of the references in two String variables.

Practice Exercise¶

The String Pool¶

The String class is special, in that we can create a String object just by putting a double-quoted literal in our code.

When we create strings this way, Java first checks to see if there's already a String with the exact same content in a special region of memory called the String Pool. If it finds one, then instead of creating a new String Java returns the reference to the existing String in the String Pool.
String s1 = "cat";
String s2 = "cat";  // There's already a "cat" in the String Pool
System.out.println(s1 == s2); // true
When we use new to create a String, then Java ignores the String Pool and creates a new object on the heap.
String c = new String("cat"); // A new object on the heap
String d = new String("cat"); // Another new String object
System.out.println(c == d);   // false
                              // The references to the two Strings are different

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