Java 中 float 或者double 類型比較大小的正確方式

一、基於閾值的比較

（1）設置一個精度e，

（2）兩數相減的結果取絕對值dif

（3）如果dif<e，兩數相等，否則不等

private static void thresholdBasedFloatsComparison()
{
    final double THRESHOLD = .0001;
 
    //Method 1
    double f1 = .0;
    for (int i = 1; i <= 11; i++) {
        f1 += .1;
    }
 
    //Method 2
    double f2 = .1 * 11;
 
    System.out.println("f1 = " + f1);
    System.out.println("f2 = " + f2);
 
    if (Math.abs(f1 - f2) < THRESHOLD)
        System.out.println("f1 and f2 are equal using threshold\n");
    else
        System.out.println("f1 and f2 are not equal using threshold\n");
}

 

二、使用BigDecimal

注意：equals  會比較兩數的精度

private static void testBdEquality(){
     BigDecimal a = new BigDecimal("2.00");
     BigDecimal b = new BigDecimal("2.0");
 
     System.out.println(a.equals(b));           // false
 
     System.out.println(a.compareTo(b) == 0);   // true
}

例子：

private static void bigDecimalComparison() {
    //Method 1
    BigDecimal f1 = new BigDecimal("0.0");
    BigDecimal pointOne = new BigDecimal("0.1");
    for (int i = 1; i <= 11; i++) {
        f1 = f1.add(pointOne);
    }
 
    //Method 2
    BigDecimal f2 = new BigDecimal("0.1");
    BigDecimal eleven = new BigDecimal("11");
    f2 = f2.multiply(eleven);
 
    System.out.println("f1 = " + f1);
    System.out.println("f2 = " + f2);
 
    if (f1.compareTo(f2) == 0)
        System.out.println("f1 and f2 are equal using BigDecimal\n");
    else
        System.out.println("f1 and f2 are not equal using BigDecimal\n");
}