String之intern()方法

叁歲伎倆 2022-09-24 01:22 251阅读 0赞

intern方法本质上来源于JVM的运行时常量池。具有动态特性，并不一定需要常量编译时才能产生，运行期间也可以把常量放入常量池。

intern 返回的是字符串对象的规范化表示形式，当调用 intern 方法时，如果常量池已经包含一个等于此 String 对象的字符串（该对象由 equals(Object) 方法确定），则返回池中的字符串。否则，将此 String 对象添加到池中，并且返回此 String 对象的引用。

package test;
    
    public class test {
    
    	public static void main(String[] args) {
    		String a = "12345";
    		String b = "12345";
    		System.out.println(a == b);
    		String c = "basy";
    		String d = new String("basy").intern();
    		String d2 = new String("basy");
    		System.out.println(c == d);
    		System.out.println(c == d2);
    		String s1 = "123";
    		String s2 = new String("123");
    		System.out.println(s1 == s2);
    		String s3 = s2.intern();
    		System.out.println(s1 == s3);
    	}
    
    }

结果：

true  
true  
false  
false  
true

附录：

/**
         * Returns a canonical representation for the string object.
         * <p>
         * A pool of strings, initially empty, is maintained privately by the
         * class {@code String}.
         * <p>
         * When the intern method is invoked, if the pool already contains a
         * string equal to this {@code String} object as determined by
         * the {@link #equals(Object)} method, then the string from the pool is
         * returned. Otherwise, this {@code String} object is added to the
         * pool and a reference to this {@code String} object is returned.
         * <p>
         * It follows that for any two strings {@code s} and {@code t},
         * {@code s.intern() == t.intern()} is {@code true}
         * if and only if {@code s.equals(t)} is {@code true}.
         * <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™ 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();