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[The{@code String} class represents character strings. All
string literals in Java programs, such as{@code "abc"}, are
implemented as instances of this class.

Strings are constant; their values cannot be changed after they are created. String buffers support mutable strings. Because String objects are immutable they can be shared. For example:

    String str = "abc";

is equivalent to:

    char data[] ={'a', 'b', 'c'};
    String str = new String(data);

Here are some more examples of how strings can be used:

    System.out.println("abc");
    String cde = "cde";
    System.out.println("abc" + cde);
    String c = "abc".substring(2,3);
    String d = cde.substring(1, 2);

The class{@code String} includes methods for examining individual characters of the sequence, for comparing strings, for searching strings, for extracting substrings, and for creating a copy of a string with all characters translated to uppercase or to lowercase. Case mapping is based on the Unicode Standard version specified by the{@link java.lang.Character Character} class.

The Java language provides special support for the string concatenation operator ( + ), and for conversion of other objects to strings. String concatenation is implemented through the{@code StringBuilder}(or{@code StringBuffer}) class and its{@code append} method. String conversions are implemented through the method {@code toString}, defined by{@code Object} and inherited by all classes in Java. For additional information on string concatenation and conversion, see Gosling, Joy, and Steele, The Java Language Specification.

Unless otherwise noted, passing a null argument to a constructor or method in this class will cause a{@link NullPointerException} to be thrown.

A{@code String} represents a string in the UTF-16 format in which supplementary characters are represented by surrogate pairs (see the section Unicode Character Representations in the{@code Character} class for more information). Index values refer to{@code char} code units, so a supplementary character uses two positions in a{@code String}.

The{@code String} class provides methods for dealing with Unicode code points (i.e., characters), in addition to those for dealing with Unicode code units (i.e.,{@code char} values).]