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javaarraysstringjvm
crazyPixel
crazyPixel

Reputation: 2330

Does java keep the String in a form of char array?

I couldn't find it on the api or by searching the web.. I know the JVM keeps every String object it has in a string pool in order to optimize memory usage. However I can't figure out how does it save it 'under the hood', Since a String is an immutable object using the toCharArray will get me a copy of the internal array the is stored on the String object in the pool? (if so than every operation involved in getting the string array as a char is O(n)) also - using charAt(i) uses the internal array of the string? or does it copy it to a new array and return the char at position i of the new copied array?

Upvotes: 1

Views: 1897

Answers (1)

malware
malware

Reputation: 393

  1. Until Java 8, Strings were internally represented as an array of characters – char[], encoded in UTF-16, so that every character uses two bytes of memory.

  2. When we create a String via the new operator, the Java compiler will create a new object and store it in the heap. For example.

    String str1= new String("Hello");

    When we create a String variable and assign a value to it, the JVM searches the pool for a String of equal value. If found, the Java compiler will simply return a reference to its memory address, without allocating additional memory.If not found, it’ll be added to the pool and its reference will be returned.

    String str2= "Hello";

  3. toCharArray() internally creates a new char[] array by copying the characters of original array to the new one.

  4. charAt(int index) returns the value of specified index of the internal (original) char[] array.

With Java 9 a new representation is provided, called Compact Strings. This new format will choose the appropriate encoding between char[] and byte[] depending on the stored content. Since the new String representation will use the UTF-16 encoding only when necessary, the amount of heap memory will be significantly lower, which in turn causes less Garbage Collector overhead on the JVM.

Source: http://www.baeldung.com/java-string-pool

Upvotes: 5

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