深入學習java源碼之ArrayList.iterator()與ArrayList.sum()
float 型： 占 4 位元組，7 位有效數字
double 型：占 8 位元組，15～16 位有效數字
Comparable接口
此接口強行對實作它的每個類的對象進行整體排序。此排序被稱為該類的自然排序 ，類的 compareTo 方法被稱為它的自然比較方法 。實作此接口的對象清單（和數組）可以通過 Collections.sort （和 Arrays.sort ）進行自動排序。實作此接口的對象可以用作有序映射表中的鍵或有序集合中的元素，無需指定比較器。 強烈推薦（雖然不是必需的）使自然排序與 equals 一緻。所謂與equals一緻是指對于類 C 的每一個 e1 和 e2 來說，當且僅當 (e1.compareTo((Object)e2) == 0) 與e1.equals((Object)e2) 具有相同的布爾值時，類 C 的自然排序才叫做與 equals 一緻 。
int compareTo(T o) 比較此對象與指定對象的順序。如果該對象小于、等于或大于指定對象，則分别傳回負整數、零或正整數。 強烈推薦 (x.compareTo(y)==0) == (x.equals(y)) 這種做法，但不是 嚴格要求這樣做。一般來說，任何實作 Comparable 接口和違背此條件的類都應該清楚地指出這一事實。推薦如此闡述：“注意：此類具有與 equals 不一緻的自然排序。”
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方法可以實作對對象數組排序，但是必須實作 Comparable接口
因為要實作對Employee對象的排序，是以在Employee類中要實作Comparable接口，也就是要實作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) //這裡比較的是什麼 sort方法實作的就是按照此比較的東西從小到大排列
return - 1 ;
if (id>other.id)
return 1 ;
return 0 ;
}
}
與Comparator的差別
Comparator位于包java.util下，而Comparable位于包java.lang下，Comparable接口将比較代碼嵌入自身類中，而後者在一個獨立的類中實作比較。 如果類的設計師沒有考慮到Compare的問題而沒有實作Comparable接口，可以通過 Comparator來實作比較算法進行排序，并且為了使用不同的排序标準做準備，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);
}
}
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java源碼
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;
}
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