货币格式化

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double 格式化输出保留两位小数

DecimalFormat

Note: 由于"四舍六入五取偶" 规则， 只能用第一种方法

# Decimal formats are generally not synchronized
new DecimalFormat("#0.00").format(0.00)   // 0.00
new DecimalFormat("#0.00").format(0.1388)   // 0.14

String.format

普通格式化，不能用于货币表示

String.format("%.2f", 0.00)   // 0.00
String.format("%.2f", -0.00)   // -0.00
String.format("%.2f", 0.439)  // 0.44

benchmark

@Test
public void benchmark() {
    int times =  1000000;
    double[] samples = new double[]{0.00, -0.00, 0.00012, 9999999.8888999e7};
    String[] res = new String[]{"0.00", "-0.00", "0.00", "99999998888999.00"};
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    for (int i = 0; i < times; ++i) {
        for (int j = 0; j < samples.length; ++ j) {
            DecimalFormat df = new DecimalFormat("#0.00");
            Assert.assertEquals(res[j], df.format(samples[j]));
        }
    }
    stopWatch.split();
    System.out.println(stopWatch.getSplitTime());
    stopWatch.reset();
    stopWatch.start();
    for (int i = 0; i < times; ++i) {
        for (int j = 0; j < samples.length; ++ j) {
            Assert.assertEquals(res[j], String.format("%.2f", samples[j]));
        }

    }
    stopWatch.split();
    System.out.println(stopWatch.getSplitTime());
    stopWatch.stop();
}