JVM 窄化类型转换指令实战

╰+哭是因爲堅強的太久メ 2022-09-03 14:20 119阅读 0赞

# 一 窄化类型转换的基本测试 #

## 1 代码 ##

// 窄化类型转换
    // 基本的使用
    public void downCast1() {
        int i = 10;
        byte b = (byte) i;
        short s = (short) i;
        char c = (char) i;
    
        long l = 10L;
        int i1 = (int) l;
        byte b1 = (byte) l;
    }
    
    public void downCast2() {
        float f = 10;
        long l = (long) f;
        int i = (int) f;
        byte b = (byte) f;
    
        double d = 10;
        byte b1 = (byte) d;
    }
    
    public void downCast3() {
        short s = 10;
        byte b = (byte) s;
    }

## 2 测试 ##

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70][]

# 二 窄化类型转换的精度损失问题 #

## 1 代码 ##

// 窄化类型转换的精度损失
    @Test
    public void downCast4() {
        int i = 128;
        byte b = (byte) i;
        System.out.println(b); // -128
    }

## 2 测试 ##

> \-128

## 3 图解 ##

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2NoZW5ncWl1bWluZw_size_16_color_FFFFFF_t_70 1][]

# 三 NAN 和 Infinity 转化测试 #

## 1 代码 ##

// 测试 NaN,无穷大 的情况
    @Test
    public void downCast5() {
        System.out.println("-------------------NAN 转 int ----------------------------------");
        double d1 = Double.NaN; // 0.0 / 0.0，它的结果是 NAN
        int i = (int) d1;
        System.out.println(d1); // NaN
        System.out.println(i); // 0 整型不存在 NaN，按 0 处理
        System.out.println("------------------Infinity 转 long 或 int-----------------------");
        double d2 = Double.POSITIVE_INFINITY; // Infinity
        long l = (long) d2;
        int j = (int) d2;
        System.out.println(l); // 9223372036854775807  转换后指向了 long 型的最大值  Infinity -> 9223372036854775807
        System.out.println(Long.MAX_VALUE); // 9223372036854775807
        System.out.println(j); // 2147483647 转换后指向了 int 型的最大值  Infinity -> 2147483647
        System.out.println(Integer.MAX_VALUE); // 2147483647
        System.out.println("-----------------Infinity 转 float----------------------------");
        float f = (float) d2;
        System.out.println(f); // Infinity   float 无穷大没有具体值
        System.out.println("-------------------NAN 转 float------------------------------");
        float f1 = (float) d1;
        System.out.println(f1); // NaN
    }

## 2 测试 ##

> NaN
> 
> 0
> 
> 9223372036854775807
> 
> 9223372036854775807
> 
> 2147483647
> 
> 2147483647
> 
> Infinity
> 
> NaN

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