Math

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public final class Math extends Object
{
	public static final double E = 2.7182818284590452354; //常量
	public static final double PI = 3.14159265358979323846;//π
	public static double abs(double a) //求绝对值
	public static double random() //返回一个0.0~1.0之间的随机数
	public static double pow(double a, double b) //返回a的b次幂
	public static double sqrt(double a) //返回a的平方根值
	public static double sin(double a) //返回a的正弦值
}

String

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public final class String extends Object implements java.io.Serializable, Comparable<String>,CharSequence
{
	private final char value[]; //字符数组，最终变量
	public String() //构造方法
	public String(String original)
	public String toString() //覆盖Object类中方法
	public int length() //返回字符串的长度
	public boolean equals(Object obj) //比较字符串是否相等
	public boolean equalsIgnoreCase (String s)//忽略字母大小写
	public int compareTo(String s) //比较字符串的大小
	public int compareToIgnoreCase(String str)
}

Integer

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public final class Integer extends Number implements Comparable<Integer>
{
	public static final int MIN_VALUE=0x80000000;//最小值-231
	public static final int MAX_VALUE = 0x7fffffff; //最大值231-1
	private final int value; //私有最终变量，构造时赋值
	public Integer(int value) //构造方法
	public Integer(String s) throws NumberFormatException
	public static int parseInt(String s) throws NumberFormatException //将字符串转换为整数，静态方法
	public String toString() //覆盖Object类中方法
	public static String toBinaryString(int i)//将i转换成二进制字符串，i≥时，省略高位0
	public static String toOctalString(int i)//将i转换成八进制字符串，i≥时，省略高位0
	public static String toHexString(int i)//将i转换成十六进制字符串
	public boolean equals(Object obj)//覆盖Object类中方法
	public int compareTo(Integer iobj)//比较两个对象值大小，返回1、0或1
}

Comparable

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public interface Comparable<T>
{
	int compareTo(T cobj) //比较对象大小
}

其中，是Comparable接口的参数，表示一个类。

举个栗子：MyDate类对象比较大小

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public class MyDate implements Comparable<MyDate>
{
	public int compareTo(MyDate d)//约定比较日期大小的规则，返回-1、0、1
	{
	if (this.year==d.year && this.month==d.month&& this.day==d.day)
		return 0;
	return (this.year>d.year || this.year==d.year &&
	this.month>d.month || this.year==d.year &&
	this.month==d.month && this.day>d.day) ? 1 : -1;
	}
}