三角函数方法:
– – – – – – public public public public public public static static static static static static double double double double double double sin(double a) cos(double a) tan(double a) asin(double a) acos(double a) atan(double a)
其中,参数a表示以弧度计量的角度。
3 Math类(续)
指数函数方法:
– public static double exp(double a) //求e的a次方 – public static double pow(double a, double b) //求a的b次方 – public static double sqrt(double a) //求a的平方根 – public static double cbrt(double a) //求a的立方根
3 Math类(续)
对数函数方法:
– public static double log(double a) //求a的自然对数lna – public static double log10(double a) //求a的以10为底的对数
3 Math类(续)
其他
常用方法:
– abs() – min()、max() – round() – random()
例3 使用Math类
源代码
4 System类
成员变量:
– public static final InputStream in – public static final PrintStream out – public static final PrintStream err
成员方法:
– public static void arraycopy(Object src,int srcPos,Object dest,int destPos,int length) – public static long currentTimeMillis() – public static void exit(int status)
5 String类
5.1 创建String对象 5.2 字符串操作
– – – – – – – – 求字符串长度 字符串比较 字符串连接 查找单个字符 查找字符串 大、小写转换 子串 其他常用方法
5.1 创建String对象
String s=new String( "This is a character strings"); String s="This is a character strings";
注意:String对象是一个常量,一旦创 注意: 建,其值不可改变。
5.2 字符串操作
求字符串长度:
String s="Hello,Java World"; int i=s.length(); //i=16
5.2 字符串操作(续)
字符串比较:
– public boolean equals(Object anObject) – public boolean equalsIgnoreCase(String anotherString) – public int compareTo(String anotherString) – public int compareToIgnoreCase(String str)
例:
boolean b1="Java".equals("java"); //b1=false //i1=-32 boolean b2="Java".equalsIgnoreCase("java"); //b2=true int i1="
Java"pareTo("java"); int i2="Java"pareToIgnoreCase("java"); //i2=0
5.2 字符串操作(续)
字符串连接:
String s1="Hello!"; String s2=s1.concat("World");
//s2="Hello!World"
String s="Hello!"+"World";
5.2 字符串操作(续)
查找单个字符:
– – – – public public public public int int int int indexOf(int ch) indexOf(int ch, int fromIndex) lastIndexOf(int ch) lastIndexOf(int ch,int fromIndex)
例:
String s="She is a student"; int i1=s.indexOf('s'); int i2=s.indexOf('s',7); int i3=s.lastIndexOf('s'); int i4=s.lastIndexOf('s',7); //i1=5 //i2=9 //i3=9 //i4=5
5.2 字符串操作(续)
查找字符串:
– – – – public public public public int int int int indexOf(String str) indexOf(String str, int fromIndex) lastIndexOf(String str) lastIndexOf(String str,int fromIndex)
例:
String s="I love
Java,I use Java!"; int i1=s.indexOf("Java"); //i1=7 int i2=s.indexOf("Java",9); //i2=18 int i3=s.lastIndexOf("Java"); //i3=18 int i4=s.lastIndexOf("Java",9); //i4=7
5.2 字符串操作(续)
大、小写转换:
– public String toLowerCase() – public String toUpperCase()
例:
String s1="Java".toLowerCase(); //s1="java" String s2="Java".toUpperCase(); //s2="JAVA"
5.2 字符串操作(续)
子串:
– public String substring(int beginIndex) – public String substring(int beginIndex,int endIndex) 例:
"unhappy".substring(2) //返回字符串"happy" "Harbison".substring(3) //返回字符串"bison" "emptiness".substring(9