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The String Pool

When we call a String constructor with new, a new String object will always be created at runtime.

String s1 = new String("Cat"); // A cat
String s2 = new String("Cat"); // Another cat
System.out.println(s1 == s2);  // false: two different references.
New String Objects

When we use string literals in our code, Java creates String objects for them in a special area of the heap: the String Pool.

  • Every part of your application shares the strings in the String Pool.

  • Only one copy of each string literal is kept in the String Pool.

String s3 = "Cat";
String s4 = "Cat";
System.out.println(s3 == s4);  // true: both references to the
                               //       same object in the String Pool.
String Pool

  • Because each String is immutable, and the values of all your string literals are known at compile time, Java is able to use the String Pool to save memory.

The term for this is interning.

  • As each String literal is encountered, Java first checks the String Pool to see if a String object with the same value already exists.

  • If so, Java just uses a reference to the existing object.

  • If not, Java creates the new String in the String Pool.

The Java compiler is smart enough to intern two concatenated string literals.

  • A string concatenation that includes a variable will not be interned, however.
String chip = "Potato" + "Chip"; // "PotatoChip" will be interned in the String Pool.
String fry = "Fry";
String french = "French" + fry;  // "FrenchFry" will NOT be interned.

String.intern()

The intern method attempts to place a String into the String Pool at runtime.

  • If a string with the same value was already in the String Pool, intern returns its reference.

System.out.println(s2 == s3);  // false: different references.
s2 = s2.intern();
System.out.println(s2 == s3);  // true: s2 now refers to the cat that was in the String Pool.
Intern - String already in String Pool

  • If it succeeds, it returns the new reference to the interned String.

String s5 = new String("Dog"); // New object on the heap.
Intern - new String

s5 = s5.intern();              // Move dog to the String Pool.
String s6 = "Dog";             // dog is already in the String Pool.
System.out.println(s5 == s6);  // true
Intern - String interned

Drill

StringAndStringBuilder/com.example.stringstringbuilder.drills.StringPoolTesting

  • Read each of the commented System.out.println statements and write down whether it will print true or false.
  • Uncomment each one and run it, making sure you understand the result.

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