深入学习java源码之Float.valueOf()与Float.sum()

深入学习java源码之ArrayList.iterator()与ArrayList.sum() 

float 型： 占 4 字节，7 位有效数字

double 型：占 8 字节，15～16 位有效数字

Comparable接口

此接口强行对实现它的每个类的对象进行整体排序。此排序被称为该类的自然排序 ，类的 compareTo 方法被称为它的自然比较方法 。实现此接口的对象列表（和数组）可以通过 Collections.sort （和 Arrays.sort ）进行自动排序。实现此接口的对象可以用作有序映射表中的键或有序集合中的元素，无需指定比较器。 强烈推荐（虽然不是必需的）使自然排序与 equals 一致。所谓与equals一致是指对于类 C 的每一个 e1 和 e2 来说，当且仅当 (e1.compareTo((Object)e2) == 0) 与e1.equals((Object)e2) 具有相同的布尔值时，类 C 的自然排序才叫做与 equals 一致 。

int compareTo(T o) 比较此对象与指定对象的顺序。如果该对象小于、等于或大于指定对象，则分别返回负整数、零或正整数。 强烈推荐 (x.compareTo(y)==0) == (x.equals(y)) 这种做法，但不是 严格要求这样做。一般来说，任何实现 Comparable 接口和违背此条件的类都应该清楚地指出这一事实。推荐如此阐述：“注意：此类具有与 equals 不一致的自然排序。”

例如

Employee[] staff =  new Employee[ 3 ];  
staff[ 0 ] =  new Employee( "harry Hacker" , 35000 );  
staff[ 1 ] =  new Employee( "carl cracke" , 75000 );  
staff[ 2 ] =  new Employee( "tony Tester" , 38000 );  
Arrays.sort(staff); //sort方法可以实现对对象数组排序，但是必须实现 Comparable接口

因为要实现对Employee对象的排序，所以在Employee类中要实现Comparable接口，也就是要实现comepareTo()方法

class Employee  implements Comparable<Employee>  
{  

private int id;  
private String name;  
private double salary;  

public Employee(String n, double s)  
    {  
        name = n;  
        salary = s;  
        Random ID =  new Random();  
        id = ID.nextInt( 10000000 );  
    }  

public int compareTo(Employee other)  
    {  
if (id<other.id) //这里比较的是什么 sort方法实现的就是按照此比较的东西从小到大排列
return - 1 ;  
if (id>other.id)  
return 1 ;  
return 0 ;  
    }  
}

与Comparator的区别

Comparator位于包java.util下，而Comparable位于包java.lang下，Comparable接口将比较代码嵌入自身类中，而后者在一个独立的类中实现比较。 如果类的设计师没有考虑到Compare的问题而没有实现Comparable接口，可以通过  Comparator来实现比较算法进行排序，并且为了使用不同的排序标准做准备，比如：升序、降序。

import java.util.TreeSet; 
import java.util.Comparator; 
class NumComparator implements Comparator<NameTag> { 
    public int compare (NameTag left,NameTag right) { 
        return(left.getNumber() - right.getNumber()); 
    } 
} 
public class CollectionNine { 
    public static void main(String arg[]) { 
        new CollectionNine(); 
    } 
    CollectionNine() { 
        NumComparator comparator = new NumComparator(); 
        TreeSet<NameTag> set = new TreeSet<NameTag>(comparator); 
        set.add(new NameTag("Agamemnon",300)); 
        set.add(new NameTag("Cato",400)); 
        set.add(new NameTag("Plato",100)); 
        set.add(new NameTag("Zeno",200)); 
        set.add(new NameTag("Archimedes",500)); 
        for(NameTag tag : set) 
            System.out.println(tag); 
    } 
}

java源码

package java.lang;

import sun.misc.FloatingDecimal;
import sun.misc.FloatConsts;
import sun.misc.DoubleConsts;

public final class Float extends Number implements Comparable<Float> {


    public static final float POSITIVE_INFINITY = 1.0f / 0.0f;

    public static final float NEGATIVE_INFINITY = -1.0f / 0.0f;

    public static final float NaN = 0.0f / 0.0f;
    
    public static final float MAX_VALUE = 0x1.fffffeP+127f; // 3.4028235e+38f   
    
    public static final float MIN_NORMAL = 0x1.0p-126f; // 1.17549435E-38f
    
    public static final float MIN_VALUE = 0x0.000002P-126f; // 1.4e-45f 
    
    public static final int MAX_EXPONENT = 127; 
    
    public static final int MIN_EXPONENT = -126;    
    
    public static final int SIZE = 32;  
    
    public static final int BYTES = SIZE / Byte.SIZE;   
    
    @SuppressWarnings("unchecked")
    public static final Class<Float> TYPE = (Class<Float>) Class.getPrimitiveClass("float");    
    
    public static String toString(float f) {
        return FloatingDecimal.toJavaFormatString(f);
    }   

    public static String toHexString(float f) {
        if (Math.abs(f) < FloatConsts.MIN_NORMAL
            &&  f != 0.0f ) {// float subnormal
            // Adjust exponent to create subnormal double, then
            // replace subnormal double exponent with subnormal float
            // exponent
            String s = Double.toHexString(Math.scalb((double)f,
                                                     /* -1022+126 */
                                                     DoubleConsts.MIN_EXPONENT-
                                                     FloatConsts.MIN_EXPONENT));
            return s.replaceFirst("p-1022$", "p-126");
        }
        else // double string will be the same as float string
            return Double.toHexString(f);
    }

    public static Float valueOf(String s) throws NumberFormatException {
        return new Float(parseFloat(s));
    }

    public static Float valueOf(float f) {
        return new Float(f);
    }

    public static float parseFloat(String s) throws NumberFormatException {
        return FloatingDecimal.parseFloat(s);
    }

    public static boolean isNaN(float v) {
        return (v != v);
    }
    
    public static boolean isInfinite(float v) {
        return (v == POSITIVE_INFINITY) || (v == NEGATIVE_INFINITY);
    }   
    
     public static boolean isFinite(float f) {
        return Math.abs(f) <= FloatConsts.MAX_VALUE;
    }
    
    private final float value;  
    
    public Float(float value) {
        this.value = value;
    }   
    
    public Float(double value) {
        this.value = (float)value;
    }   
    
    public Float(String s) throws NumberFormatException {
        value = parseFloat(s);
    }   
    
    public boolean isNaN() {
        return isNaN(value);
    }   
    
    public boolean isInfinite() {
        return isInfinite(value);
    }   

    public String toString() {
        return Float.toString(value);
    }
    
    public byte byteValue() {
        return (byte)value;
    }   
    
    public short shortValue() {
        return (short)value;
    }

    public int intValue() {
        return (int)value;
    }

    public long longValue() {
        return (long)value;
    }

    public float floatValue() {
        return value;
    }

    public double doubleValue() {
        return (double)value;
    }

    @Override
    public int hashCode() {
        return Float.hashCode(value);
    }

    public static int hashCode(float value) {
        return floatToIntBits(value);
    }

    public boolean equals(Object obj) {
        return (obj instanceof Float)
               && (floatToIntBits(((Float)obj).value) == floatToIntBits(value));
    }

    public static int floatToIntBits(float value) {
        int result = floatToRawIntBits(value);
        // Check for NaN based on values of bit fields, maximum
        // exponent and nonzero significand.
        if ( ((result & FloatConsts.EXP_BIT_MASK) ==
              FloatConsts.EXP_BIT_MASK) &&
             (result & FloatConsts.SIGNIF_BIT_MASK) != 0)
            result = 0x7fc00000;
        return result;
    }

    public static native int floatToRawIntBits(float value);


    public static native float intBitsToFloat(int bits);

    public int compareTo(Float anotherFloat) {
        return Float.compare(value, anotherFloat.value);
    }

    public static int compare(float f1, float f2) {
        if (f1 < f2)
            return -1;           // Neither val is NaN, thisVal is smaller
        if (f1 > f2)
            return 1;            // Neither val is NaN, thisVal is larger

        // Cannot use floatToRawIntBits because of possibility of NaNs.
        int thisBits    = Float.floatToIntBits(f1);
        int anotherBits = Float.floatToIntBits(f2);

        return (thisBits == anotherBits ?  0 : // Values are equal
                (thisBits < anotherBits ? -1 : // (-0.0, 0.0) or (!NaN, NaN)
                 1));                          // (0.0, -0.0) or (NaN, !NaN)
    }

    public static float sum(float a, float b) {
        return a + b;
    }

    public static float max(float a, float b) {
        return Math.max(a, b);
    }

    public static float min(float a, float b) {
        return Math.min(a, b);
    }
    private static final long serialVersionUID = -2671257302660747028L;
}

package java.lang;
import java.util.*;

public interface Comparable<T> {

    public int compareTo(T o);
}

package java.lang;

public abstract class Number implements java.io.Serializable {

    public abstract int intValue();

    public abstract long longValue();

    public abstract float floatValue();

    public abstract double doubleValue();

    public byte byteValue() {
        return (byte)intValue();
    }

    public short shortValue() {
        return (short)intValue();
    }
    
    private static final long serialVersionUID = -8742448824652078965L;
}