字符串池为Java中的相同String创建两个字符串对象

Question

可能重复:
Java字符串和StringPool

我创建了两个字符串

String s1="MyString";
String s2=new String("MyString");
System.out.println(s1==s2);

它打印"false".我们知道字符串池不会为同一个字符串文字创建两个对象.

那么这里发生了什么？它在字符串池中为相同的字符串文字"MyString"创建了两个不同的字符串对象(文字).

我知道 equals()方法true在这里返回.

但是当我们使用==它时应该比较两个引用,它们应该引用String常量池中的相同String对象.

为什么即使找到匹配项,它也不会引用String池中的现有String对象？

Answer 1

第一个进入池,而第二个存储在堆上.

用s2 = s2.intern();它来返回true.

当你做一个intern()对字符串时,JVM确保该字符串是存在于池中.如果它尚不存在,则在池中创建.否则,返回已存在的实例.我认为这解释了这种==行为.

String s1="MyString";
String s2=new String("MyString");
s2 = s2.intern();
System.out.println(s1==s2);

作为参考,这里是String.intern()文档说的:

/**
 * Returns a canonical representation for the string object.
 * <p>
 * A pool of strings, initially empty, is maintained privately by the
 * class <code>String</code>.
 * <p>
 * When the intern method is invoked, if the pool already contains a
 * string equal to this <code>String</code> object as determined by
 * the {@link #equals(Object)} method, then the string from the pool is
 * returned. Otherwise, this <code>String</code> object is added to the
 * pool and a reference to this <code>String</code> object is returned.
 * <p>
 * It follows that for any two strings <code>s</code> and <code>t</code>,
 * <code>s.intern()&nbsp;==&nbsp;t.intern()</code> is <code>true</code>
 * if and only if <code>s.equals(t)</code> is <code>true</code>.
 * <p>
 * All literal strings and string-valued constant expressions are
 * interned. String literals are defined in section 3.10.5 of the
 * <cite>The Java&trade; Language Specification</cite>.
 *
 * @return  a string that has the same contents as this string, but is
 *          guaranteed to be from a pool of unique strings.
 */
public native String intern();

Answer 2

在String s2=new String("MyString");你正在创建一个新实例的行中String,所以它肯定不会是同一个实例s1.

如果你这样做:

System.out.println(s1=="MyString");

你会得到的true.