blocks|key|939840|text|正如James在他的评论中指出的那样，这是因为这就是2的补码的工作方式。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|939841|换句话说，你可以表示2%5E8+=+256种值。在本例中，它被用作128个负数、127个正数和0。如果我们使用7位来表示值，%2B1位来表示一个符号，我们可以少表示一个值，并且也会有两个零(这将是非常不幸的，因为比较两个值将会因为此而更加复杂)。|939842|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

As James pointed out in his comment, it's because that's how two's complement works.

If we put it in other terms, you can represent 2^8 = 256 kind of values. which is, in this case used as 128 negative numbers, 127 positive numbers, and zero. If we used 7 bits to represent the value, +1 bit for a sign, we could represent one less value and would also have two zeros (which would be very unfortunate as comparing two values would be more complicated because of that).

blocks|key|939897|text|在java中，所有像byte，short，int，long，float，double这样的变量都是有符号的。所以很简单，头位总是指定什么是(负或正)，但是因为数字可以被2整除，所以一半被移位为负，默认情况下0是正的。所以看起来是这样的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|939898|这是积极的|939899|%2B%7C0001001|939900|1%7C0001001|939901|这是负面的|939902|-%7C0001001|939903|0%7C0001001|939904|作为一个字节的短小负数是|939905|-000000011111111|939906|0000000011111111|939907|entityMap^0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|V|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|W|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|X|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|Y|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|Z|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|10|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|11|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|12|8|@]|9|@]|A|$]]|$1|P|3|Q|5|6|7|13|8|@]|9|@]|A|$]]|$1|R|3|S|5|6|7|14|8|@]|9|@]|A|$]]|$1|T|3|-4|5|6|7|15|8|@]|9|@]|A|$]]]|U|$]]

in java all the variables like byte short int long float double are written as signed .
so is very simple the head bit always specifies what is( negative or positive), but because the numbers are dividable by 2 half is shifted as negative , 0 is positive by default .
so it looks like this :

this is positive 
+|0001001 
1|0001001 
this is negative 
-|0001001 
0|0001001 
as a byte 
short a negative is 
-000000011111111 
0000000011111111

blocks|key|1154295|text|基本数字类型可以表示2%5En个数字。看一个case+n=2。你可以表示四种情况，让我们称它们为a，b，c，d。然后你可以同意a=-2,+b=-1,+c=0,+d=1+(这是可接受的方式)或a=-1,+b=0,+c=1,+d=2+(可能，但不使用)。因此，如果你只有一个零，并且保持2%5En个状态，那么增加n的abs(min)+!=+max会移动边界，但abs(min)+!=+max仍然有效。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1154296|entityMap^0|1P|K|2L|J|46|1|48|F|4U|F|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|G|$]]

Basic numeric types can represent 2^n numbers. Look at a case n=2. You can represent four cases, lets call them a, b, c, d. Then you can agree to either <code>a=-2, b=-1, c=0, d=1</code> (this is accepted way) or <code>a=-1, b=0, c=1, d=2</code> (Possible, but not used). So, if you only have one zero and hold 2^n states your <code>abs(min) != max</code> Increasing the <code>n</code> moves the borders, but <code>abs(min) != max</code> still holds.

blocks|key|940056|text|字节由8位组成->1位符号(正或负)7位值|type|unstyled|depth|inlineStyleRanges|entityRanges|data|940057|因此，范围-2%5E7负(-128+)到2%5E7+-1正(127)|940058|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

byte consist of 8 bit ---> 1 bit sign (positive or negative ) 7 bit value 

so the range -2^7 negative (-128 ) to 2^7 -1 positive(127)

blocks|key|940137|text|不涉及2的补码:+2%5E8+(因为一个字节是8位数字，可以有2个值中的1个)=+256，所以一个字节可以表示的最大值是256。因此，表示从-128到-1的数字是我们范围的一半。我认为这里的问题是为什么最大正值是127而不是128。这是因为我们必须表示数字0，所以包含0-127是我们范围内的其他128种可能性。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|940138|如果我们只允许正值，比如一个无符号字节，其中负数是不可能的，那么范围将是0-255，因为这是256个不同的值(包括0)。|940139|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Without getting into two's complement:
2^8 (since a byte is 8 digits and can have 1 of 2 values) = 256, so the most individual values a byte can represent is 256. 
so, representing the numbers -128 to -1 is half our range. 
I believe the question here is why is the max positive value 127 rather than 128. This is because we have to represent the number 0, so inclusively 0-127 is the other 128 possibilities of our range.

If we were only allowing positive values, such as an unsigned byte where negative numbers aren't possible, the range would be 0-255, since these are 256 different values (including the 0).

blocks|key|940209|text|在接受2的数字恭维之后，我们总是留下一个表示额外数字的状态，所以我们将该状态改为-128。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|940210|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

after taking two's compliment of number we always left with one state of representing number extra so we turn that state to -128.

I don't understand why the lowest value a byte can take is <code>-128</code>. I can see that the highest value is <code>127</code>, because it's <code>01111111</code> in binary, but how does one represent <code>-128</code> with only 8 bits, one of which is used for the sign? Positive 128 would already be 8-bit, i.e. <code>10000000</code>, and then you would need a 9th bit to represent the negative sign. 

Could someone please help explain this to me.

Why is the range of bytes -128 to 127 in Java?

我不明白为什么一个字节可以接受的最低值是-128。我可以看到最高的值是127，因为它是二进制的01111111，但是如何表示只有8位的-128，其中一位用于符号？正128应该已经是8位了，也就是10000000，然后你需要第9位来表示负号。有人能帮我解释一下吗。

问为什么Java中的字节范围是从128到127？
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