一.intern方法

1.定義

/**
 * 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();
 

根據原始碼的解釋，該方法返回常量池中對應的字串，若常量池中存在該字串，直接返回該物件引用，若不存在，將該字串物件加入常量池然後再返回其引用。

注意： 字串常量在jdk1.7之前存在方法區，jdk1.7之後移到了堆中。

2.應用

見部落格https://www.cnblogs.com/Kidezyq/p/8040338.html

二.String中面試涉及到的問題

1.JDK 6和JDK 7中substring的原理及區別

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

2.String對“+”的過載

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