String源码解析二

一.intern方法

1.定义

/**
 * 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();

根据源码的解释，该方法返回常量池中对应的字符串，若常量池中存在该字符串，直接返回该对象引用，若不存在，将该字符串对象加入常量池然后再返回其引用。

注意： 字符串常量在jdk1.7之前存在方法区，jdk1.7之后移到了堆中。

2.应用

见博客https://www.cnblogs.com/Kidezyq/p/8040338.html

二.String中面试涉及到的问题

1.JDK 6和JDK 7中substring的原理及区别

https://blog.csdn.net/renfufei/article/details/14058047

2.String对“+”的重载

https://blog.csdn.net/codejas/article/details/78662146