JDK源码 -- Integer
2019-08-19 本文已影响0人
TomyZhang
一、概念
类定义：
public final class Integer extends Number implements Comparable<Integer>
- 继承Number抽象类，表明是个数字类型。
- 实现Comparable接口，可进行比较。
特点：
- 不可继承。
- 不可变。
二、使用
//TestInteger
public class TestInteger {
private static final String TAG = "TestInteger";
private Integer integer = new Integer(64);
public void testEquals() {
Log.d(TAG, "zwm, equals: " + integer.equals(64));
}
public void testHashCode() {
Log.d(TAG, "zwm, hashCode: " + integer.hashCode());
}
public void testByteValue() {
byte value = integer.byteValue();
Log.d(TAG, "zwm, byteValue: " + value);
}
public void testShortValue() {
short value = integer.shortValue();
Log.d(TAG, "zwm, shortValue: " + value);
}
public void testIntValue() {
int value = integer.intValue();
Log.d(TAG, "zwm, intValue: " + value);
}
public void testLongValue() {
long value = integer.longValue();
Log.d(TAG, "zwm, longValue: " + value);
}
public void testFloatValue() {
float value = integer.floatValue();
Log.d(TAG, "zwm, floatValue: " + value);
}
public void testDoubleValue() {
double value = integer.doubleValue();
Log.d(TAG, "zwm, doubleValue: " + value);
}
public void testCompareTo() {
Log.d(TAG, "zwm, compareTo: " + integer.compareTo(66));
}
public void testToString() {
Log.d(TAG, "zwm, toStrng: " + integer.toString());
}
public void test() {
Integer a = new Integer(100); //在堆内存生成一个新对象
Integer b = new Integer(100); //在堆内存生成一个新对象
Log.d(TAG, "zwm, a == b: " + (a==b));
Integer c = new Integer(100); //在堆内存生成一个新对象
int d = 100; //基本数据类型
Log.d(TAG, "zwm, c == d: " + (c==d)); //Integer与int进行比较时，Integer自动拆箱为int
Integer e = new Integer(100); //在堆内存生成一个新对象
Integer f = 100; //Integer f = Integer.valueOf(100)，在[-128,127]区间，返回IntegerCache缓存
Log.d(TAG, "zwm, e == f: " + (e==f));
Integer g = 100; //在[-128,127]区间，返回IntegerCache缓存
Integer h = 100; //在[-128,127]区间，返回IntegerCache缓存
Log.d(TAG, "zwm, g == h: " + (g==h));
Integer i = 128; //不在[-128,127]区间，在堆内存生成一个新对象
Integer j = 128; //不在[-128,127]区间，在堆内存生成一个新对象
Log.d(TAG, "zwm, i == j: " + (i==j));
}
public void testStaticMethod() {
Integer value = Integer.valueOf("123");
Log.d(TAG, "zwm, valueOf 123: " + value);
int value2 = Integer.parseInt("123");
Log.d(TAG, "zwm, parseInt 123: " + value2);
String str = Integer.toString(66);
Log.d(TAG, "zwm, toString 66: " + str);
String str2 = Integer.toBinaryString(66);
Log.d(TAG, "zwm, toBinaryString 66: " + str2);
String str3 = Integer.toOctalString(66);
Log.d(TAG, "zwm, toOctalString 66: " + str3);
String str4 = Integer.toHexString(66);
Log.d(TAG, "zwm, toHexString 66: " + str4);
}
}
//测试代码
private void testMethod() {
Log.d(TAG, "zwm, testMethod");
TestInteger testInteger = new TestInteger();
testInteger.testEquals();
testInteger.testHashCode();
testInteger.testByteValue();
testInteger.testShortValue();
testInteger.testIntValue();
testInteger.testLongValue();
testInteger.testFloatValue();
testInteger.testDoubleValue();
testInteger.testCompareTo();
testInteger.testToString();
testInteger.test();
testInteger.testStaticMethod();
}
//输出log
2019-08-21 11:45:23.025 zwm, testMethod
2019-08-21 11:45:23.026 zwm, equals: true
2019-08-21 11:45:23.026 zwm, hashCode: 64
2019-08-21 11:45:23.027 zwm, byteValue: 64
2019-08-21 11:45:23.027 zwm, shortValue: 64
2019-08-21 11:45:23.027 zwm, intValue: 64
2019-08-21 11:45:23.027 zwm, longValue: 64
2019-08-21 11:45:23.027 zwm, floatValue: 64.0
2019-08-21 11:45:23.027 zwm, doubleValue: 64.0
2019-08-21 11:45:23.028 zwm, compareTo: -1
2019-08-21 11:45:23.028 zwm, toStrng: 64
2019-08-21 11:45:23.028 zwm, a == b: false
2019-08-21 11:45:23.028 zwm, c == d: true
2019-08-21 11:45:23.028 zwm, e == f: false
2019-08-21 11:45:23.028 zwm, g == h: true
2019-08-21 11:45:23.028 zwm, i == j: false
2019-08-21 11:45:23.028 zwm, valueOf 123: 123
2019-08-21 11:45:23.029 zwm, parseInt 123: 123
2019-08-21 11:45:23.029 zwm, toString 66: 66
2019-08-21 11:45:23.029 zwm, toBinaryString 66: 1000010
2019-08-21 11:45:23.029 zwm, toOctalString 66: 102
2019-08-21 11:45:23.029 zwm, toHexString 66: 42
三、原理
重要参数
//用于实际存放数据
private final int value;
//最小值
public static final int MIN_VALUE = 0x80000000;
//最大值
public static final int MAX_VALUE = 0x7fffffff;
//表明Integer类为基本数据类型int的包装器类型
public static final Class<Integer> TYPE = (Class<Integer>) Class.getPrimitiveClass("int");
构造函数
//指定一个int类型数据的构造函数
public Integer(int value) {
this.value = value;
}
//指定一个字符串类型数据的构造函数
public Integer(String s) throws NumberFormatException {
this.value = parseInt(s, 10);
}
public boolean equals(Object obj)
//比较基本类型数据是否相等
public boolean equals(Object obj) {
if (obj instanceof Integer) {
return value == ((Integer)obj).intValue();
}
return false;
}
public static int hashCode(int value)
//返回基本类型数据
public static int hashCode(int value) {
return value;
}
public int intValue()
//返回int型数据
public int intValue() {
return value;
}
public byte byteValue()
//返回byte型数据
public byte byteValue() {
return (byte)value;
}
public int compareTo(Integer anotherInteger)
//将当前Integer对象与指定的Integer对象进行比较
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);
}
public String toString()
//将Integer对象转成字符串对象
public String toString() {
return toString(value);
}
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];
getChars(i, size, buf);
return new String(buf, true);
}
static int stringSize(int x) {
for (int i=0; ; i++)
if (x <= sizeTable[i])
return i+1;
}
private static class IntegerCache
//缓存类
//在第一次被使用的时候进行初始化，可支持-128到127之间的自动装箱过程，最大值127可以通过JVM的启动参数-XX:AutoBoxCacheMax=size修改
private static class IntegerCache {
static final int low = -128;
static final int high;
static final Integer cache[]; //缓存数组
static { //静态块
// high value may be configured by property
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++); //初始化缓存数组
// range [-128, 127] must be interned (JLS7 5.1.7)
assert IntegerCache.high >= 127;
}
private IntegerCache() {}
}
public static Integer valueOf(int i)
//将int型数据转成Integer对象
//如果在缓存范围内则返回缓存对象，否则创建新的Integer对象
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}
public static int parseInt(String s)
//将字符串转成十进制int型数据
public static int parseInt(String s) throws NumberFormatException {
return parseInt(s,10);
}
public static String toBinaryString(int i)
//将int型数据转成二进制字符串
public static String toBinaryString(int i) {
return toUnsignedString0(i, 1);
}
private static String toUnsignedString0(int val, int shift) {
// assert shift > 0 && shift <=5 : "Illegal shift value";
int mag = Integer.SIZE - Integer.numberOfLeadingZeros(val);
int chars = Math.max(((mag + (shift - 1)) / shift), 1);
char[] buf = new char[chars]; //创建char型数组
formatUnsignedInt(val, shift, buf, 0, chars);
// Use special constructor which takes over "buf".
return new String(buf, true);
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 {
buf[offset + --charPos] = Integer.digits[val & mask];
val >>>= shift;
} while (val != 0 && charPos > 0);
return charPos;
}
