java.lang中的常用类.ppt

JAVASE,java.lang中的常用类,本章学习目标,了解java.lang 包 掌握包装类 掌握String和StringBuffer类 运用类Math、Class和Object类,包装类简介,包装类-1,int num = 10 ;,原始数 据类型,原始数据类型声明的变量,对象,不是对象,是对象,怎么变 成对象,java.lang提供,原始数据类型,包装类,包装类-2,包装类-3,public class LangDemo public static void main(String args) Boolean objBool = new Boolean(true); Character objChar = new Character(X); Integer objInt = new Integer(100); Long objLong = new Long(2568); Double objDou = new Double(3.1415); System.out.println(objBool); System.out.println(objChar); System.out.println(objInt); System.out.println(objLong); System.out.println(objDou); 注：,包装类的静态方法 valueOf()，用于将字符串转成相应包装类的对象。,public class LangDemo public static void main(String args) String str = “128“; /如果转换失败，将会引发NumberFormatException异常 /引发失败的原因：1。数值超过范围 2.字符串不是表示数值 Byte objByte = Byte.valueOf(str); Short objShort = Short.valueOf(str); Integer objInt = Integer.valueOf(str); Long objLong = Long.valueOf(str); System.out.println(objByte); System.out.println(objShort); System.out.println(objInt); System.out.println(objLong); ,注： 除了Boolean和Character 类以外，其他的包装类都有静态的parseXXX()方法。（XXX指代具体的数据类型）,public class ParseTest public static void main(String args) String str = “116“; int i = Integer.parseInt(str); short s = Short.parseShort(str); byte b = Byte.parseByte(str); long l = Long.parseLong(str); float f = Float.parseFloat(str); double d = Double.parseDouble(str); System.out.println(i); System.out.println(s); System.out.println(b); System.out.println(l); System.out.println(f); System.out.println(d); ,范例演示,包装类-4,包装类-5,public class CharacterDemo public static void main(String args) char charArray = *, 7, b, , A; for (int i = 0; i charArray.length; i+) if (Character.isDigit(charArrayi) System.out.println(charArrayi + “是一个数字。“); if (Character.isLetter(charArrayi) System.out.println(charArrayi + “是一个字母。“); if (Character.isWhitespace(charArrayi) System.out.println(charArrayi + “是一个空格。“); if (Character.isLowerCase(charArrayi) System.out.println(charArrayi + “是小写形式。“); if (Character.isUpperCase(charArrayi) System.out.println(charArrayi + “是大写形式。“); ,范例演示,String类,String 类,字符串字面量,对象,未修改的原始字符串,使用 String 类的方法,可以更改字符串版本,原始字符串保持不变,String类构造方法,范例演示,public class StringDemo public static void main(String args) char aryChar = N, e, w, F,u,t,u,r,e; String str1 = “NewFuture“; /利用一个字符串常量值创建新的字符串 String str2 = new String(“NewFuture“); /利用一个字符型数组创建新的字符串 String str3 = new String(aryChar); System.out.println(str1); System.out.println(str2); System.out.println(str3); ,Storage String pool & stack & heap String pool（字符串池）: JVM has a String pool, it saves a lot of String objects that created by shorthand initializer. Data sharing Stack （栈） : Store primitive data（char、byte、short、int、long、float、double、boolean）and constructor. Data sharing. heap（堆）：Store Object.,Constructing Strings,Format: String newString = new String(stringLiteral); String message = “stringLiteral“;,Example: String message = new String(“Hello“); String message = “Welcome to Java“;,String str=new String(“abc“);,Question: How many String objects are created?,Two,Constructor: public String(String original) /other code . ,Note:The parameter is a String object,String a= “abc“; String b= “abc“;,Difference between new and shorthand initializer,Question: How many String objects are created?,One,String类字符串长度,String str = “John Smith”,长度是？,使用length()方法获取,哦，str的长度是“10”,public class StringDemo public static void main(String args) String str1 = “John Smith“; String str2 = new String(“I Love Java“); System.out.println(str1.length(); System.out.println(str2.length(); ,String类字符串比较1,字符串 1,字符串 2,由 equals() 方法确定,同一个对象,用 = 运算符检查,检查字符串是否指向同一个或不同的对象,检查组成字符串内容的字符,public class StringDemo public static void main(String args) public static void main(String args) String str1 = “学习新技术把握新未来“; String str2 = “学习新技术把握新未来“; String str3 = new String(“学习新技术把握新未来“); String str4 = new String(“学习新技术把握新未来“); if (str1 = str2) System.out.println(“str1和str2指向同一字符串“); else System.out.println(“str1和str2分别指向不同字符串“); if (str1.equals(str2) System.out.println(“str1和str2的内容完全相同“); else System.out.println(“str1和str2的内容不相同“); if (str3 = str4) System.out.println(“str3和str4指向同一字符串“); else System.out.println(“str3和str4分别指向不同字符串“); if (str3.equals(str4) System.out.println(“str3和str4的内容完全相同“); else System.out.println(“str3和str4的内容不相同“); ,String类-字符串比较2,String类字符串搜索,public class StringDemo public static void main(String args) String strEmail = ““; int index; System.out.println(“E-mail地址：“ + strEmail); index = strEmail.indexOf(); System.out.println(“字符出现的索引：“ + index); index = strEmail.indexOf(“sun“); System.out.println(“字符串“sun“出现的索引：“ + index); index = strEmail.lastIndexOf(a); System.out.println(“a字符最后一次出现的索引：“ + index); ,String类字符串提取,public class StringDemo public static void main(String args) String str1 = “ Java is OOP“; String str2 = new String(“NewFuture“); System.out.println(str1.charAt(2); System.out.println(str1.substring(5); System.out.println(str1.substring(2, 9); System.out.println(str1.concat(str2); System.out.println(str1 + str2); System.out.println(str1.replace(a, e); System.out.println(str1.trim(); ,public String substring(int beginIndex) Returns a new string that is a substring of this string. The substring begins at the specified index and extends to the end of this string. public String substring(int beginIndex, int endIndex) Returns a new string that is a substring of this string. The substring begins at the specified beginIndex and extends to the character at index endIndex - 1. Parameters: the beginning index, inclusive. Returns: the specified substring. Throws: StringIndexOutOfBoundsException if the beginIndex is out of range.,String s1 = “Welcome to Java“; String s2 = s1.substring(0, 11) + “HTML“;,String类更改字符串大小写,public class StringDemo public static void main(String args) String str1 = “Java is OOP“; String str2; str2 = str1.toLowerCase(); System.out.println(str2); str2 = str1.toUpperCase(); System.out.println(str2); ,StringBuffer类-1,StringBuffer类用于表示可以修改的字符串。 使用连接运算符(+)与字符串一起使用自动创建StringBuffer对象。,StringBuffer类-2,StringBuffer类-3,public class StringBufferDemo public static void main(String args) StringBuffer strBuf = new StringBuffer(“Java“); strBuf.append(“ Guide Ver1/“); System.out.println(strBuf); strBuf.append(3); System.out.println(strBuf); strBuf.insert(5, “Student“); System.out.println(strBuf); strBuf.setCharAt(20, .); System.out.println(strBuf); strBuf.reverse(); System.out.println(strBuf); String str = strBuf.toString(); System.out.println(str); ,范例演示,不可变性-1,创建,String类,直接修改,StringBuffer类,与String同等的类,表示可增加和可编写的字符序列,可以将字符插入到中间位置或将字符附加到末端,解决方案,The StringBuffer class is an alternative to the String class. In general, a string buffer can be used wherever a string is used. StringBuffer is more flexible than String. You can add, insert, or append new contents into a string buffer. However, the value of a String object is fixed once the string is created.,StringBuffer类-1,不可变性-2,public class Immutablity public static void main(String args) String str1=“Java“; /创建一个String类的实例变量 String str2=str1; /保存一个引用副本 /创建一个StringBuffer类的实例变量 StringBuffer stringBuffer1=new StringBuffer(“NewFuture“); /保存一个StringBuffer对象引用副本 StringBuffer stringBuffer2=stringBuffer1; System.out.println (“在改变之前:“); System.out.println (“(str1=str2)：“+(str1=str2); System.out.println (“(stringBuffer1=stringBuffer2)：“+(stringBuffer1=stringBuffer2); /改变值 str1=str1+“ is OOP“; stringBuffer1=stringBuffer1.append(“study Java“); System.out.println (“在改变之后:“); System.out.println (“(str1=str2)：“+(str1=str2); System.out.println (“str1=“+str1); System.out.println (“str2=“+str2); System.out.println (“(stringBuffer1=stringBuffer2)：“+(stringBuffer1=stringBuffer2); System.out.println (“stringBuffer1=“+stringBuffer1); System.out.println (“stringBuffer2=“+stringBuffer2); ,范例演示,Math类,Math类,静 态 方 法,基本数学运算,几何运算,Math类方法,public class MathDemo /* 构造方法 */ protected MathDemo() /* main 方法演示 Math 类的不同方法 * param args 传递至 main 方法的参数 */ public static void main(String args) /* 此变量存储 num 的值*/ int num = 38; /* 该变量存储 num1 的值*/ float num1 = 65.7f; System.out.println(Math.ceil(num); System.out.println(Math.ceil(num1); System.out.println(Math.floor(num); System.out.println(Math.floor(num1); System.out.println(Math.round(num); System.out.println(Math.round(num1); ,范例演示,Object类-1,是其他所有类的共同父类。 用户类没有扩展自其他类，默认扩展自Object类。,Object类-2,class Student private String mName; private int mAge; public Student(String name, int age) /构造方法 mName = name; mAge = age; /重写Object类中的toString方法 public String toString() String str = “姓名：“ + mName + “, 年龄：“ + mAge + “岁“; return (str); public class ToStringDemo public static void main(String args) Student std = new Student(“张三“, 18); System.out.println(std); /默认调用toString方法 ,范例演示,自动创建对象,Class类-1,无需声明,Class对象,怎么创建呢？,通过对象的getClass()方法,使用Class的静态方法forName(),Class类-2,范例演示,public class ClassDemo public static void main(String args) try /*使用forName方法获得任意一个类的类描述对象 这里以StringBuffer类为例 forName方法有可能抛异常，必须捕捉*/ Class cls = Class.forName(“jav