Java中String.format和+运算符对比

在拼接字符串时我们通常会有两种方法，一种是用String类重载‘+’运算符进行拼接，本质上是调用StringBuilder.append。还有一种类似C的printf风格，调用String.format(),底层是调用formatter.format方法。那从性能和内存占用两个方面考虑到底是哪一个方法更好呢？

做了一个小小的实验.注意测试的时候注释掉非测试方法，免得内存占用会被其他的方法影响.

class StringTest {

    public static void main(String[] args) {
       //  testOperatorPlus();
        //testAppend();
       testFormat();
    }

    private static void testFormat() {
        Runtime runtime = Runtime.getRuntime();
        long memory;
        long prev_time;
        int i;
        long time;
        StringBuilder sb = new StringBuilder();
        memory = runtime.freeMemory();
        prev_time = System.currentTimeMillis();
        for (i = 0; i < 10000; i++) {
            String s = String.format("Blah %d Blah %d Blah %d", i, i, i);
        }
        long ww=runtime.freeMemory();
        time = System.currentTimeMillis() - prev_time;
        memory = memory - ww;
        System.out.println("Time: " + time + "    Memory Usage: " + memory);
    }

    private static void testAppend() {
        Runtime runtime = Runtime.getRuntime();
        long memory;
        long prev_time;
        int i;
        long time;
        StringBuilder sb = new StringBuilder();
        memory = runtime.freeMemory();
        prev_time = System.currentTimeMillis();
        for (i = 0; i < 10000; i++) {
            sb.append("Blah ");
            sb.append(i);
            sb.append("Blah ");
            sb.append(i);
            sb.append("Blah ");
            sb.append(i);
        }
        time = System.currentTimeMillis() - prev_time;
        memory = memory - runtime.freeMemory();
        System.out.println("Time: " + time + "    Memory Usage: " + memory);
    }

    private static void testOperatorPlus() {
        Runtime runtime = Runtime.getRuntime();
        long memory;
        long prev_time;
        int i;
        long time;
        StringBuilder sb = new StringBuilder();
        memory = runtime.freeMemory();
        prev_time = System.currentTimeMillis();
        for (i = 0; i < 1000000; i++) {
            String s = "Blah " + i + "Blah " + i + "Blah " + i;
        }
        time = System.currentTimeMillis() - prev_time;
        memory = memory - runtime.freeMemory();
        System.out.println("Time: " + time + "    Memory Usage: " + memory);
    }
}

结果如下：

Method	Time(ms)	Memory Usage(long)
‘+’ operator	115	28805400
StringBuilder.append	14	884768
String.foramt	276	23175080

可以看到StringBuilder.append的执行时间和内存占用都是最优的。'+'运算符比直接调用StringBuilder.append要慢上不少，特别是要连接的字符串数量较多时，内存占用也特别大。String.format由于每次都有生成一个Formatter对象，较慢也是情理之中。

PS：测试数据少且缺少对比实验，结论适用性有限，仅供参考。
另见：http://stackoverflow.com/q/925423/5032462