java - 为什么(byte)128等于-128

Question

    System.out.println((byte) 128);//-128

Answer 1

It is caused by the automatic transformation of java. The principle is as follows:
1. 128 is an int integer with 32 bits, the first 24 are all 0 and the last 8 bits are 1000 0000
2. (byte)128 is the first after being transformed into byte. is 1, Java considers it as the complement identifier of a negative number
3. When System.out.println is called, the Java type system will automatically convert the byte type to int. At this time, a signed left shift operation is performed, and the first 24 All bits are 1, and the last 8 bits are 1000 0000, which is still -128.
4, so the output is -128

tip: Java should perform automatic type conversion when doing byte operations, and it does not support unsigned integers. Pay special attention to it. Usually you need to use the & operation to block the wrong bits caused by automatic expansion

Answer 2

The value range of byte is -128~127, 128 overflows

Answer 3

1. 128 是一个 int 类型整数 00000000 00000000 00000000 10000000, 长度为 32 Bits

2. byte 类型整数长度为 8 位, 所以强制转换后为最后 8 位 10000000

3. The first bit from left to right is the sign bit, 0 时值为 0~127，1 时值为 -128~-1

4. Call System.out.println(int) 时 Java 把 byte -128 转回 int -128

5. So (byte) 128 == (int) -128