What is the difference between "text" and new String("text")?

new String("text"); explicitly creates a new and referentially distinct instance of a String object; String s = "text"; may reuse an instance from the string constant pool if one is available.

You very rarely would ever want to use the new String(anotherString) constructor. From the API:

String(String original) : Initializes a newly created String object so that it represents the same sequence of characters as the argument; in other words, the newly created string is a copy of the argument string. Unless an explicit copy of original is needed, use of this constructor is unnecessary since strings are immutable.

What referential distinction means

Examine the following snippet:

    String s1 = "foobar";
    String s2 = "foobar";

    System.out.println(s1 == s2);      // true

    s2 = new String("foobar");
    System.out.println(s1 == s2);      // false
    System.out.println(s1.equals(s2)); // true

== on two reference types is a reference identity comparison. Two objects that are equals are not necessarily ==. It is usually wrong to use == on reference types; most of the time equals need to be used instead.

Nonetheless, if for whatever reason you need to create two equals but not == string, you can use the new String(anotherString) constructor. It needs to be said again, however, that this is very peculiar, and is rarely the intention.

References

JLS 15.21.3 Reference Equality Operators == and !=
class Object - boolean Object(equals)

Related issues

Java String.equals versus ==
How do I compare strings in Java?

String literals will go into String Constant Pool.

The below snapshot might help you to understand it visually to remember it for longer time.

enter image description here

Object creation line by line:

String str1 = new String("java5");

Using string literal "java5" in the constructor, a new string value is stored in string constant pool. Using new operator, a new string object is created in the heap with "java5" as value.

String str2 = "java5"

Reference "str2" is pointed to already stored value in string constant pool

String str3 = new String(str2);

A new string object is created in the heap with the same value as reference by "str2"

String str4 = "java5";

Reference "str4" is pointed to already stored value in string constant pool

Total objects : Heap - 2, Pool - 1

JLS

The concept is called "interning" by the JLS.

Relevant passage from JLS 7 3.10.5:

Moreover, a string literal always refers to the same instance of class String. This is because string literals - or, more generally, strings that are the values of constant expressions (§15.28) - are "interned" so as to share unique instances, using the method String.intern.

Example 3.10.5-1. String Literals

The program consisting of the compilation unit (§7.3):
package testPackage;
class Test {
    public static void main(String[] args) {
        String hello = "Hello", lo = "lo";
        System.out.print((hello == "Hello") + " ");
        System.out.print((Other.hello == hello) + " ");
        System.out.print((other.Other.hello == hello) + " ");
        System.out.print((hello == ("Hel"+"lo")) + " ");
        System.out.print((hello == ("Hel"+lo)) + " ");
        System.out.println(hello == ("Hel"+lo).intern());
    }
}
class Other { static String hello = "Hello"; }
and the compilation unit:
package other;
public class Other { public static String hello = "Hello"; }
produces the output:
true true true true false true

JVMS

JVMS 7 5.1 says:

A string literal is a reference to an instance of class String, and is derived from a CONSTANT_String_info structure (§4.4.3) in the binary representation of a class or interface. The CONSTANT_String_info structure gives the sequence of Unicode code points constituting the string literal.

The Java programming language requires that identical string literals (that is, literals that contain the same sequence of code points) must refer to the same instance of class String (JLS §3.10.5). In addition, if the method String.intern is called on any string, the result is a reference to the same class instance that would be returned if that string appeared as a literal. Thus, the following expression must have the value true:
("a" + "b" + "c").intern() == "abc"
To derive a string literal, the Java Virtual Machine examines the sequence of code points given by the CONSTANT_String_info structure.

If the method String.intern has previously been called on an instance of class String containing a sequence of Unicode code points identical to that given by the CONSTANT_String_info structure, then the result of string literal derivation is a reference to that same instance of class String.

Otherwise, a new instance of class String is created containing the sequence of Unicode code points given by the CONSTANT_String_info structure; a reference to that class instance is the result of string literal derivation. Finally, the intern method of the new String instance is invoked.

Bytecode

It is also instructive to look at the bytecode implementation on OpenJDK 7.

If we decompile:

public class StringPool {
    public static void main(String[] args) {
        String a = "abc";
        String b = "abc";
        String c = new String("abc");
        System.out.println(a);
        System.out.println(b);
        System.out.println(a == c);
    }
}

we have on the constant pool:

#2 = String             #32   // abc
[...]
#32 = Utf8               abc

and main:

 0: ldc           #2          // String abc
 2: astore_1
 3: ldc           #2          // String abc
 5: astore_2
 6: new           #3          // class java/lang/String
 9: dup
10: ldc           #2          // String abc
12: invokespecial #4          // Method java/lang/String."<init>":(Ljava/lang/String;)V
15: astore_3
16: getstatic     #5          // Field java/lang/System.out:Ljava/io/PrintStream;
19: aload_1
20: invokevirtual #6          // Method java/io/PrintStream.println:(Ljava/lang/String;)V
23: getstatic     #5          // Field java/lang/System.out:Ljava/io/PrintStream;
26: aload_2
27: invokevirtual #6          // Method java/io/PrintStream.println:(Ljava/lang/String;)V
30: getstatic     #5          // Field java/lang/System.out:Ljava/io/PrintStream;
33: aload_1
34: aload_3
35: if_acmpne     42
38: iconst_1
39: goto          43
42: iconst_0
43: invokevirtual #7          // Method java/io/PrintStream.println:(Z)V

Note how:

0 and 3: the same ldc #2 constant is loaded (the literals)
12: a new string instance is created (with #2 as argument)
35: a and c are compared as regular objects with if_acmpne

The representation of constant strings is quite magic on the bytecode:

it has a dedicated CONSTANT_String_info structure, unlike regular objects (e.g. new String)
the struct points to a CONSTANT_Utf8_info Structure that contains the data. That is the only necessary data to represent the string.

and the JVMS quote above seems to say that whenever the Utf8 pointed to is the same, then identical instances are loaded by ldc.

I have done similar tests for fields, and:

static final String s = "abc" points to the constant table through the ConstantValue Attribute
non-final fields don't have that attribute, but can still be initialized with ldc

Conclusion: there is direct bytecode support for the string pool, and the memory representation is efficient.

Bonus: compare that to the Integer pool, which does not have direct bytecode support (i.e. no CONSTANT_String_info analogue).

What is the difference between "text" and new String("text")?

Related questions

What referential distinction means

References

Related issues

JLS

JVMS

Bytecode

Related

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