1)声明部:
public final class Integer extends Number implements Comparable<Integer>
extends Number, 重写方法:
public byte byteValue() { return (byte)value; } public short shortValue() { return (short)value; } public int intValue() { return value; } public long longValue() { return (long)value; } public float floatValue() { return (float)value; } public double doubleValue() { return (double)value; }
implements Comparable<T>,接口实现如下:
public int compareTo(Integer anotherInteger) { return compare(this.value, anotherInteger.value); } public static int compare(int x, int y) { return (x < y) ? -1 : ((x == y) ? 0 : 1); }
与实现该接口无关观察compareUnsigned方法:
compareUnsigned 例子:
Integer a1 = 0b00000000000000000000000000000001; Integer a2 = 0b00000000000000000000000000000011; Integer a3 = 0b10000000000000000000000000000001; Integer a4 = 0b10000000000000000000000000000011; int r1 = Integer.compareUnsigned(a1.intValue(),a2.intValue()); int r2 = Integer.compareUnsigned(a1.intValue(),a3.intValue()); int r3 = Integer.compareUnsigned(a3.intValue(),a4.intValue()); Out.println("a1,a2,a3,a4分别为:" + a1 +"," + a2 + "," + a3 + "," + a4); Out.println("r1,r2,r3分别为:" + r1 +"," + r2 + "," + r3);
result:
a1,a2,a3,a4分别为:1,3,-2147483647,-2147483645 r1,r2,r3分别为:-1,-1,-1
算法分析:比较是无符号比较方法,而默认是有符号整型,所以需要特殊方法处理。先看Integer的范围:
0B10000000000000000000000000000000
0B10000000000000000000000000000001
..
0B11111111111111111111111111111111
…
0B00000000000000000000000000000000
0B00000000000000000000000000000001
0B00000000000000000000000000000010
…
0B01111111111111111111111111111111
在同一个符号下:除符号位其他位的数字越大,那么该数字越大。+MIN.VALUE就是修改符号位。不影响同一个符号位下的大小比较,
假设a=-1,b=1按无符号考虑:a>b;
按有符合考虑:a<b,通过修改符号位,a+MIN.VALUE(正数)>b+MIN.VALUE(负数)
实现了无符号比较。
2)属性
@Native public static final int MIN_VALUE = 0x80000000; @Native public static final int MAX_VALUE = 0x7fffffff; /** * The {@code Class} instance representing the primitive type {@code int}. */ public static final Class<Integer> TYPE = (Class<Integer>) Class.getPrimitiveClass("int"); /** * All possible chars for representing a number as a String */ final static char[] digits = { '0' , '1' , '2' , '3' , '4' , '5' , '6' , '7' , '8' , '9' , 'a' , 'b' , 'c' , 'd' , 'e' , 'f' , 'g' , 'h' , 'i' , 'j' , 'k' , 'l' , 'm' , 'n' , 'o' , 'p' , 'q' , 'r' , 's' , 't' , 'u' , 'v' , 'w' , 'x' , 'y' , 'z' }; //十位上的数值,可以根据36,取的十位上数为3 final static char [] DigitTens = { '0', '0', '0', '0', '0', '0', '0', '0', '0', '0', '1', '1', '1', '1', '1', '1', '1', '1', '1', '1', '2', '2', '2', '2', '2', '2', '2', '2', '2', '2', '3', '3', '3', '3', '3', '3', '3', '3', '3', '3', '4', '4', '4', '4', '4', '4', '4', '4', '4', '4', '5', '5', '5', '5', '5', '5', '5', '5', '5', '5', '6', '6', '6', '6', '6', '6', '6', '6', '6', '6', '7', '7', '7', '7', '7', '7', '7', '7', '7', '7', '8', '8', '8', '8', '8', '8', '8', '8', '8', '8', '9', '9', '9', '9', '9', '9', '9', '9', '9', '9', } ; //个位上的数值,可以根据36,取的个位上数为6 final static char [] DigitOnes = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', } ; final static int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,99999999, 999999999, Integer.MAX_VALUE }; //避免除法操作,直接比较大小返回位数 static int stringSize(int x) { for (int i=0; ; i++) if (x <= sizeTable[i]) return i+1; } public Integer(int value) { this.value = value;} @Native public static final int SIZE = 32; public static final int BYTES = SIZE / Byte.SIZE;
3)内部私有类
private static class IntegerCache { static final int low = -128; static final int high; static final Integer cache[]; static { <span style="color:#ff0000;">// high value may be configured by property</span> int h = 127; String integerCacheHighPropValue = sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high"); if (integerCacheHighPropValue != null) { try { int i = parseInt(integerCacheHighPropValue); i = Math.max(i, 127); // Maximum array size is Integer.MAX_VALUE h = Math.min(i, Integer.MAX_VALUE - (-low) -1); } catch( NumberFormatException nfe) { // If the property cannot be parsed into an int, ignore it. } } high = h; cache = new Integer[(high - low) + 1]; int j = low; for(int k = 0; k < cache.length; k++) cache[k] = new Integer(j++); // range [-128, 127] must be interned (JLS7 5.1.7) assert IntegerCache.high >= 127; } private IntegerCache() {} }
和Short.java和Byte.java一样,都是通过静态代码块初始化缓存对象数组,不过,IntegerCache对象的high可以通过JVM的启动参数设置,缺省为127。
4)初始化方法
构造函数:
public Integer(String s) throws NumberFormatException { this.value = parseInt(s, 10); } public Integer(int value) { this.value = value; }
其他方法:
public static int parseInt(String s, int radix); public static int parseInt(String s) throws NumberFormatException; public static Integer valueOf(String s, int radix) throws NumberFormatException; public static Integer valueOf(String s) throws NumberFormatException; public static Integer valueOf(int i); public static int parseUnsignedInt(String s) throws NumberFormatException; public static int parseUnsignedInt(String s, int radix);
观察“其他方法”的代码,难点主要在下面的方法:
public static int parseInt(String s, int radix) throws NumberFormatException { /* * WARNING: This method may be invoked early during VM initialization * before IntegerCache is initialized. Care must be taken to not use * the valueOf method. */ if (s == null) { throw new NumberFormatException("null"); } if (radix < Character.MIN_RADIX) { throw new NumberFormatException("radix " + radix + " less than Character.MIN_RADIX"); } if (radix > Character.MAX_RADIX) { throw new NumberFormatException("radix " + radix + " greater than Character.MAX_RADIX"); } int result = 0; boolean negative = false; int i = 0, len = s.length(); int limit = -Integer.MAX_VALUE; int multmin; int digit; if (len > 0) { char firstChar = s.charAt(0); if (firstChar < '0') { // Possible leading "+" or "-" if (firstChar == '-') { negative = true; limit = Integer.MIN_VALUE; } else if (firstChar != '+') throw NumberFormatException.forInputString(s); if (len == 1) // Cannot have lone "+" or "-" throw NumberFormatException.forInputString(s); i++; } multmin = limit / radix;//设不同进制下的极限值 while (i < len) { <span style="color:#ff0000;"> // Accumulating negatively avoids surprises near MAX_VALUE //该方法返回该字符根据进制对应的数字,比如2进制1那么返回1,比如16进制F那么返回15 digit = Character.digit(s.charAt(i++),radix);//i++:0->1 if (digit < 0) { throw NumberFormatException.forInputString(s); } if (result < multmin) { throw NumberFormatException.forInputString(s); } result *= radix; if (result < limit + digit) { throw NumberFormatException.forInputString(s); } result -= digit;</span> } } else { throw NumberFormatException.forInputString(s); } return negative ? result : -result; }
该代码段例子分析:
F1F(16进制)
(1)digit 15;result *= radix -》0;result -= digit -》 -15
(2)digit 1;result *= radix -》-15*16;result -= digit -》 -15*16 – 1(相当于累加)
(3)digit 15;result *= radix -》-(15*16+1)*16;result -= digit -》 -(15*16+1)*16 – 15
(4)-( -(15*16+1)*16 – 15)= (15*15+1)*16 + 15
public static int parseUnsignedInt(String s, int radix) throws NumberFormatException { if (s == null) { throw new NumberFormatException("null"); } int len = s.length(); if (len > 0) { char firstChar = s.charAt(0); if (firstChar == '-') { throw new NumberFormatException(String.format("Illegal leading minus sign " + "on unsigned string %s.", s)); } else { if (len <= 5 || // Integer.MAX_VALUE in Character.MAX_RADIX is 6 digits (radix == 10 && len <= 9) ) { // Integer.MAX_VALUE in base 10 is 10 digits return parseInt(s, radix); } else { long ell = Long.parseLong(s, radix); if ((ell & 0xffff_ffff_0000_0000L) == 0) { return (int) ell; } else { throw new NumberFormatException(String.format("String value %s exceeds " + "range of unsigned int.", s)); } } } } else { throw NumberFormatException.forInputString(s); } }
例子:
Integer a11 = 3; Integer a6 = Integer.parseInt("11",2); Integer a7 = Integer.parseUnsignedInt("80000000",16); Integer a9 = Integer.valueOf(3); Integer a10 = new Integer(3); boolean b1 = a6==a9;//true boolean b2 = a6==a11;//true boolean b3 = a6==a10;//false
初始化对象进行使用parseInt或者valueOf方法,如果值在IntegerCache范围内,可以直接获取对象。
5)一些toXXXString方法:
public static String toString(int i, int radix) { if (radix < Character.MIN_RADIX || radix > Character.MAX_RADIX) radix = 10; /* Use the faster version */ if (radix == 10) { return toString(i); } char buf[] = new char[33];//二进制 boolean negative = (i < 0); int charPos = 32; if (!negative) { i = -i; } while (i <= -radix) { buf[charPos--] = digits[-(i % radix)]; i = i / radix; } buf[charPos] = digits[-i]; if (negative) { buf[--charPos] = '-'; } //从buf数组的charPos位置开始截取33-charPos字符转换为String类型 return new String(buf, charPos, (33 - charPos)); }
分析该函数之前,我们用十进制转换为二进制来解释下算法:比如10,10%2 商5余0;5%2 商2余1;2%2 商1余0;2%1 商0余1,把余数倒序拼接1010,1010就是二进制。i++号在后面的意思是先赋值然后自身加1;++i在前面的是先自身加1后赋值;–同样。
public static String toUnsignedString(int i, int radix); public static String toHexString(int i); public static String toOctalString(int i); public static String toBinaryString(int i){ return toUnsignedString0(i, 1); } //前面4个方法都是调用该方法,shift:次幂 private static String toUnsignedString0(int val, int shift) { // assert shift > 0 && shift <=5 : "Illegal shift value"; //获取有效的二进制的位数 31位二进制 int mag = Integer.SIZE - Integer.numberOfLeadingZeros(val); //一个算法,可以根据二进制的有效位数,求出8进制或者16进制的位数 //比如0100 0000 shift=3 -》 (7+(3-1))/3 = 3 八进制需要3位表示 int chars = Math.max(((mag + (shift - 1)) / shift), 1); char[] buf = new char[chars]; formatUnsignedInt(val, shift, buf, 0, chars); // Use special constructor which takes over "buf". return new String(buf, true); } //转换为2进制,高位-》低位,直至遇到1停止,0的个数 //比如0000 0000 0000 0000 0000 0000 0000 0001-》31 //比如0100 0000 0000 0000 0000 0000 0000 0001-》1 //比如0010 0000 0000 0000 0000 0000 0000 0001-》2 public static int numberOfLeadingZeros(int i) { // HD, Figure 5-6 if (i == 0) return 32; int n = 1; if (i >>> 16 == 0) { n += 16; i <<= 16; } if (i >>> 24 == 0) { n += 8; i <<= 8; } if (i >>> 28 == 0) { n += 4; i <<= 4; } if (i >>> 30 == 0) { n += 2; i <<= 2; } n -= i >>> 31; return n; } static int formatUnsignedInt(int val, int shift, char[] buf, int offset, int len) { int charPos = len; int radix = 1 << shift; int mask = radix - 1; do { //val&mask 可以获取末尾使用该进制表示的数值 //例子 假设16进制 // 1010 1010 1010 1000 //& 0000 0000 0000 1111 // 0000 0000 0000 1000 buf[offset + --charPos] = Integer.digits[val & mask]; val >>>= shift; } while (val != 0 && charPos > 0); return charPos; }
public static String toString(int i) { if (i == Integer.MIN_VALUE) return "-2147483648"; int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i); char[] buf = new char[size]; // 将Integer数读入到char[]数组 getChars(i, size, buf); return new String(buf, true); } static void getChars(int i, int index, char[] buf) { int q, r; int charPos = index; char sign = 0; if (i < 0) { sign = '-'; i = -i; } // Generate two digits per iteration // 处理超过2的16次方的大数 while (i >= 65536) { q = i / 100; // really: r = i - (q * 100); // 假设 65536 那么等于 36,根据36取的十位数和个位数上的数值 r = i - ((q << 6) + (q << 5) + (q << 2)); //655 i = q; //个位 6 buf [--charPos] = DigitOnes[r]; //十位 3 buf [--charPos] = DigitTens[r]; } // Fall thru to fast mode for smaller numbers // assert(i <= 65536, i); // 处理<2的16次方的大数 for (;;) { q = (i * 52429) >>> (16+3);//i*52429/524288 ≈0.1000003。相当于i/10 //获取个位的数字 r = i - ((q << 3) + (q << 1)); // r = i-(q*10) ... buf [--charPos] = digits [r]; i = q; if (i == 0) break; } if (sign != 0) { buf [--charPos] = sign; } }
一个例子:
for(int i = 0;i<65536;i++){ int q = (i * 52429) >>> (16+3);//相当于q = i/10; int j = i/10; if(q != j){ Out.println(false); } } 输出:true