Java中Integer的最大值和最小值

Question

从JDK1.0开始，Integer中就定义了MIN_VALUE和MAX-VALUE两个常量：

/**
 * A constant holding the minimum value an {@code int} can
 * have, -2<sup>31</sup>.
 */
public static final int   MIN_VALUE = 0x80000000;

/**
 * A constant holding the maximum value an {@code int} can
 * have, 2<sup>31</sup>-1.
 */
public static final int   MAX_VALUE = 0x7fffffff;

Q1：谁能给解释一下,这两个常量为什么会分别定义成0x80000000和0x7fffffff。
Q2：java.lang.String的最大长度是多少？
Q3：如下代码能抛出异常吗？为什么

int x = Integer.MAX_VALUE+10;
if(x >= Integer.MAX_VALUE || x <= Integer.MIN_VALUE){ //throw exception}

Answer 1

Q1:

The signed number of the four-byte integer is -2^31~2^31-1 (also written in the comments of MIN_VALUE and MAX-VALUE). It will be clear after checking the two's complement binary representation of the corresponding boundary

Q2:

I don’t use Java much, so I’m not sure. It should be very long. Attached are the key fields of String

   /** The value is used for character storage. */
    private final char value[];

    /** The offset is the first index of the storage that is used. */
    private final int offset;

    /** The count is the number of characters in the String. */
    private final int count;

    /** Cache the hash code for the string */
    private int hash; // Default to 0

Q3:

int x = Integer.MAX_VALUE 10; x overflows, x is actually -2^31 9, the if condition is not true, and no exception will be thrown.

Answer 2

• Q1: What the computer can understand is the binary string of 01. The binary representation of numerical values ​​within the computer includes positive code, complement code and complement code. Generally, two's complement is used for representation and operations. MIN_VALUE = 0x80000000 and MAX_VALUE = 0x7fffffff are the minimum value (-2^31) and maximum value (2^31-1) of Integer expressed in complement. As for why the two's complement representation is used, simply speaking, it is to facilitate calculations. For details, you can Google it yourself or look up this basic textbook. As for why the maximum and minimum values ​​of Integer are these two numbers, this is because the Java language specification stipulates that the int type is 4 bytes, whether it is a 32/64-bit machine, this is the basic part of its claimed cross-platform.

• Q2: The maximum length of a String depends on its internal data representation. String is represented internally by a char array. The length of the array is limited in Java to the maximum value that can be represented by an int type, which is MAX_VALUE = 0x7fffffff in Q1. This is reflected by its internal properties representing offset int offset and length int count.

• Q3: The above code will not throw an exception. For values ​​outside the representation range, the strategy adopted is the truncation effect, that is, the low bits are directly intercepted and the high bit information that is out of the range is discarded. This is the so-called overflow. For example, if the int type operation result exceeds the representation range, the lower 32 bits (4 bytes in Q1) are directly intercepted as the operation result. As a result, the first line of the above code will overflow, and the overflow result will cause the condition of the second line to be false.