Heap Memory and String Interning

This example demonstrates the concept of String interning in Java and how it affects heap memory. String interning is a mechanism for reusing String objects by storing only one copy of each distinct String value in a special memory area called the String pool.

Code Snippet

In this code, 'str1' and 'str2' both reference the same String literal "Hello". Because of String interning, they point to the same object in the String pool (a part of the heap). 'str3' is created using the 'new' keyword, which creates a new String object on the heap, even though its value is "Hello". Therefore, 'str1' and 'str3' are different objects. 'str4' is created using 'new String("Hello")' and then '.intern()', which checks if a String with the value "Hello" exists in the String pool. If it does, it returns the reference to that object; otherwise, it adds "Hello" to the pool and returns the reference. This causes 'str1' and 'str4' to refer to the same object in the String pool. Thus, the == operator checks if both variables reference the same object on the heap.

public class StringInterning {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "Hello";
        String str3 = new String("Hello");
        String str4 = new String("Hello").intern();

        System.out.println("str1 == str2: " + (str1 == str2)); // true
        System.out.println("str1 == str3: " + (str1 == str3)); // false
        System.out.println("str1 == str4: " + (str1 == str4)); // true
    }
}

Concepts Behind the Snippet

• String Pool: A special area in the heap memory where String literals are stored. When a String literal is encountered (e.g., "Hello"), the JVM checks if a String with that value already exists in the String pool. If it does, the existing String object is reused. Otherwise, a new String object is created in the String pool.
• String Interning: The process of adding a String object to the String pool. The String.intern() method returns a canonical representation for the String object. If the pool already contains a String equal to this String object as determined by the equals(Object) method, then the String from the pool is returned. Otherwise, this String object is added to the pool and a reference to this String object is returned.
• Immutability of Strings: Strings in Java are immutable, meaning their value cannot be changed after they are created. This property makes String interning safe and efficient.

Real-Life Use Case

String interning can be beneficial in applications that handle a large number of duplicate String values, such as web applications that process user input or applications that read data from files. By interning Strings, you can reduce memory consumption and improve performance, especially when comparing Strings frequently. Configuration management and reading of metadata, which use the same string keys over and over, is a good use case.

Best Practices

• Use String interning judiciously. While it can save memory, the intern() method has some overhead. Avoid interning every String.
• Consider the trade-offs between memory savings and CPU time when deciding whether to use String interning.
• String interning is most effective when you have a large number of duplicate String values.

Interview Tip

Be prepared to discuss String interning and its impact on memory usage and performance. Understand the difference between creating Strings with literals and using the new keyword. Be able to explain when and why you would use String interning. Also, know that prior to Java 7, the String pool was located in the PermGen space (part of the permanent generation), which had a fixed size. As of Java 7, the String pool is located in the heap, which is dynamically resized.

When to use them

Use String interning when you have a high frequency of duplicated strings and memory is limited. This is especially relevant in legacy systems with limited RAM or when processing extremely large datasets. Modern JVMs are highly optimized so the performance gains might be negligible or even negative in some cases.

Memory footprint

String interning reduces the memory footprint by sharing the same String object across multiple references. Without interning, each duplicate String would consume its own memory space on the heap. With interning, only one copy of the String is stored, saving memory. The impact on memory footprint depends on the number of duplicate strings and the length of these strings.

Alternatives

Alternatives to String interning include using a custom String pool or using a more memory-efficient data structure for storing strings, such as a Trie. Consider using libraries like Guava's ImmutableSet for storing a set of unique strings efficiently.

Pros

• Reduced memory consumption: By sharing String objects.
• Improved performance: String comparisons (using '==') become faster because you are comparing references, not the content of the strings.

Cons

• Overhead: Interning can add overhead, especially if the String pool gets large.
• Potential for memory leaks: If you intern strings that are not frequently reused, the String pool can grow without bound, potentially leading to memory leaks.