在拼接字符串時我們通常會有兩種方法，一種是用String類重載‘+’運算符進行拼接，本質上是調用StringBuilder.append。還有一種類似C的printf風格，調用String.format(),底層是調用formatter.format方法。那從性能和內存占用兩個方面考慮到底是哪一個方法更好呢？

做了一個小小的實驗.注意測試的時候注釋掉非測試方法，免得內存占用會被其他的方法影響.

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);
    }
}

結果如下：

可以看到StringBuilder.append的執(zhí)行時間和內存占用都是最優(yōu)的。'+'運算符比直接調用StringBuilder.append要慢上不少，特別是要連接的字符串數量較多時，內存占用也特別大。String.format由于每次都有生成一個Formatter對象，較慢也是情理之中。

PS：測試數據少且缺少對比實驗，結論適用性有限，僅供參考。
另見：http://stackoverflow.com/q/925423/5032462