JAVA基础1(代码剖析)

代码选自善学堂JAVA培训

</pre><pre class="html" name="code">public class TestBinaryNum {public static void main(String[] args){int a = 0b0000_0000_0000_0000_0000_0000_0000_0011;int b = 1_2312_3131;System.out.println(a);System.out.println(b);}}

/** * 测试JDK7中的switch新特性 * @author dell * */public class TestSwitch02 {public static void main(String[] args) {String  a = "高琪";switch (a) {   //JDK7的新特性，表达式结果可以是字符串！！！case "马士兵":System.out.println("输入的马士兵");break;case "高琪":System.out.println("输入的高琪");break;default:System.out.println("大家好！");break;}}}

/** * 测试内部类的使用 * @author dell * */public class Outer {public static void main(String[] args) {Face f = new Face();Face.Nose n = f.new Nose();n.breath();Face.Ear e = new Face.Ear();e.listen();}}class Face {int type;String shape="瓜子脸";static String color="红润";class Nose {void breath(){System.out.println(shape); System.out.println(Face.this.type);System.out.println("呼吸！");}}static class Ear {void listen(){System.out.println(color); System.out.println("我在听！"); }}}

继承1：

public class Animal {String str;public void voice(){System.out.println("普通动物叫声！");}}class Cat extends Animal {public void voice(){System.out.println("喵喵喵");}public void catchMouse(){System.out.println("抓老鼠");}}public class Test {public static void testAnimalVoice(Animal c){c.voice();if(c instanceof Cat){                   //instance((Cat) c).catchMouse();}}public static void main(String[] args) {Animal a = new Cat();Cat a2 = (Cat)a;testAnimalVoice(a);}}

多态：

public class HttpServlet {public void service(){System.out.println("HttpServlet.service()");this.doGet();}public void doGet(){System.out.println("HttpServlet.doGet()");}}public class MyServlet extends HttpServlet {public void doGet(){System.out.println("MyServlet.doGet()");}}public class Test {public static void main(String[] args) {HttpServlet s = new MyServlet();s.service();  //这里service里边调用的是 myservlet里边的doGet方法}}

输出：HttpServlet.service()
           MyServlet.doGet()

静态代码初始化块：在类一开加载的时候就执行 只执行一次

public class TestStaticInitBlock extends Parent001 {static int a ;static {System.out.println("静态初始化TestStaticInitBlock!");a = 100;}public static void main(String[] args) {}}public class Parent001 /*extends Object*/ {static int aa;static {System.out.println(" 静态初始化Parent001");aa=200;}}

输出： 静态初始化Parent001
           静态初始化TestStaticInitBlock!

final

public /*final*/ class Animal {    //final修饰类则说明，这个类不能被继承！public /*final*/ void run(){   //final加到方法前面，意味着该方法不能被子类重写！System.out.println("跑跑！");}}class Bird  extends Animal {public void run(){super.run();System.out.println("我是一个小小小小鸟，飞呀飞不高");}}public class TestFinal {public static void main(String[] args) {final int MAX_VALUE= 200;    //常量。double d = Math.PI;}}

this关键字：

public class Student { String name; int id;     public  Student(String name,int id){ this(name);   //相当于调用下边的构造方法

              //通过this调用其他构造方法，必须位于第一句！ Constructor call must be the first statement in a constructor this.id = id; }  public Student(String name){ this.name = name; }  public void setName(String name){ this.name = name; }  public void study(){ this.name=  "张三"; System.out.println(name+"在學習"); } }

重载

public class TestOverload {public int add(int a, int b){return a+b;}public static void main(String[] args) {MyMath m = new MyMath();int result = m.add(4.2,8);System.out.println(result);}}class  MyMath {int a;int b;public MyMath(){}public MyMath(int a){this.a = a;}public MyMath(int b, int a){this.b = b;this.a = a;}public int add(int b, double a){return (int)(a+b);}public int add(double a, int b){return (int)(a+b);}public int add(int a, int b){return a+b;}public int add(int a, int b,int c){return a+b+c;}}

数组：

class Car {String name;public Car(String name){this.name=  name;}}/** * 1. 数组是相同数据类型(数据类型可以为任意类型)的有序集合 * 2. 数组也是对象。数组元素相当于对象的成员变量(详情请见内存图) * 3. 数组长度的确定的，不可变的。如果越界，则报：ArrayIndexOutofBoundsException */public class Test02 {public static void main(String[] args) {//声明int[] a;int b[];//创建数组对象a = new int[4];b = new int[5];Car[] cars1 = new Car[4];cars1[0] = new Car("奔驰");Car c2 = new Car("奔驰");//初始化(对数组元素的初始化)//默认初始化：数组元素相当于对象的成员变量，默认值跟成员变量的规则一样。数字0，布尔false,char\u0000,引用：null//动态初始化：for(int i=0;i<a.length;i++){a[i] = i*12;}//静态初始化int c[] = {23,43,56,78};   //长度：4，索引范围：[0,3]Car[] cars = {new Car("奔驰"),new Car("比亚迪"),new Car("宝马")};System.out.println(c2==cars[0]); //false}}

/** * 模拟实现JDK中提供的ArrayList类 * @author dell * */public class MyArrayList {/**     * The value is used for object storage.     */    private Object[] value;    /**     * The size is the number of objects used.     */    private int size;        public MyArrayList(){    //value = new Object[16];    this(10);    }    public MyArrayList(int size){    if(size<0){    try {throw new Exception();  //手动抛出一个异常。 讲到异常章节再说，先混个眼熟} catch (Exception e) {e.printStackTrace();}    }    value = new Object[size];    }        public int size(){    return size;    }        public boolean isEmpty() {        return size == 0;    }        public void add(Object obj){    value[size] = obj;    size++;    if(size>=value.length){    //装不下了。扩容吧！    int newCapacity = value.length*2;    Object[] newList = new Object[newCapacity];//    System.arraycopy(src, srcPos, dest, destPos, length);        for(int i=0;i<value.length;i++){    newList[i] = value[i];    }        value = newList;    }    }        public Object get(int index){    rangeCheck(index);        return value[index];    }        public int indexOf(Object obj){    if(obj==null){    return -1;    }else{    for(int i=0;i<value.length;i++){    if(obj==value[i]){    return i;    }    }    return -1;    }    }        public int lastIndexOf(Object obj){    if(obj==null){    return -1;    }else{    for(int i=value.length-1;i>=0;i--){    if(obj==value[i]){    return i;    }    }    return -1;    }    }        public Object set(int index, Object object) {    rangeCheck(index);    Object old = value[index];        value[index] = object;        return old;    }        public void rangeCheck(int index){    if(index<0||index>size-1){    //[0,size-1]    try {throw new Exception();  //手动抛出一个异常。 讲到异常章节再说，先混个眼熟} catch (Exception e) {e.printStackTrace();}    }    }            public static void main(String[] args) {MyArrayList  list = new MyArrayList(2);list.add("aaa");list.add(new Human("高琪"));list.add("bbbb");list.add("bbbb");list.add("bbbb");list.add("bbbb");ArrayList list2;Human h = (Human) list.get(1);System.out.println(h.getName());System.out.println(list.size());}    }class Human {private String name;public Human(String name) {super();this.name = name;}public String getName() {return name;}public void setName(String name) {this.name = name;}}

字符串常用方法

public class Test02 {public static void main(String[] args) {String str = new String("abcd");String str2 = new String("abcd");System.out.println(str2.equals(str));   //比较内容是否相等。System.out.println(str2==str);  //注意这里是falseSystem.out.println(str.charAt(2));String str3 = "def";String str4 = "def";System.out.println(str3.equals(str4));System.out.println(str3==str4); //注意这里是true 因为这种方式和上边的方式不同 这种是字符串常量 只有一份System.out.println(str3.indexOf('y')); String s = str3.substring(0);System.out.println(s);String str5 = str3.replace('e', '*');System.out.println(str5);String str6 = "abcde,rrtt,cccee";String[] strArray = str6.split(",");for(int i=0;i<strArray.length;i++){System.out.println(strArray[i]);}String str7 = "  aa  bb  ";String str77 = str7.trim();System.out.println(str77.length()); System.out.println("Abc".equalsIgnoreCase("abc"));System.out.println("Abcbd".indexOf('b')); System.out.println("Abcbd".lastIndexOf('b')); System.out.println("Abcbd".startsWith("Ab")); System.out.println("Abcbd".endsWith("bd")); System.out.println("Abcbd".toLowerCase()); System.out.println("Abcbd".toUpperCase()); System.out.println("##################");String gh = new String("a");for (int i = 0; i < 1000; i++) {gh = gh + i;}System.out.println(gh); }}

/** * 测试可变字符序列。StringBuilder(线程不安全，效率高),StringBuffer(线程安全，效率低) * String：不可变字符序列 * @author dell * */public class Test01 {public static void main(String[] args) {StringBuilder sb = new StringBuilder();   //字符数组长度初始为16StringBuilder sb1 = new StringBuilder(32);   //字符数组长度初始为32StringBuilder sb2 = new StringBuilder("abcd");   //字符数组长度初始为32, value[]={'a','b','c','d',\u0000,\u0000...}sb2.append("efg");sb2.append(true).append(321).append("随便");   //通过return this实现方法链.System.out.println(sb2);System.out.println("##################");StringBuilder gh = new StringBuilder("a");for (int i = 0; i < 1000; i++) {gh.append(i);}System.out.println(gh);}}

Integer  d = 1234;Integer  d2 = 1234;System.out.println(d==d2);          //falseSystem.out.println(d.equals(d2));  //trueSystem.out.println("###################"); Integer d3 = -100;    //[-128,127]之间的数，仍然当做基本数据类型来处理。Integer d4 = -100;System.out.println(d3==d4);      //trueSystem.out.println(d3.equals(d4));  //true