toString方法
public class test {
public static void main(String[] args) {
Teacher t=new Teacher();
System.out.println("1 "+t+" 2 "+t.toString());
}
}
class Teacher {
public String toString() {
return "I am a teacher";
}
}
输出：
1 I am a teacher 2 I am a teacher
说明：
public String toString() {return "I am a teacher";}不能写成大写的public String ToString！
重写必须要COPY过来！
toString
public String toString()
- 返回该对象的字符串表示。通常，
toString方法会返回一个“以文本方式表示”此对象的字符串。结果应是一个简明但易于读懂的信息表达式。建议所有子类都重写此方法。Object类的toString方法返回一个字符串，该字符串由类名（对象是该类的一个实例）、at 标记符“@”和此对象哈希码的无符号十六进制表示组成。换句话说，该方法返回一个字符串，它的值等于：getClass().getName() + '@' + Integer.toHexString(hashCode())
-
- 返回：
- 该对象的字符串表示形式。
认识一下toString方法，并且重写父类方法！
equals方法：
public class test {
public static void main(String[] args) {
Cat c1=new Cat(1,2,3);
Cat c2=new Cat(1,2,3);
System.out.println(c1==c2);
}
}
class Cat {
int color;
int height,weight;
public Cat(int color,int height,int weight) {
this.color=color;
this.height=height;
this.weight=weight;
}
}
//永远不等，比较的是地址
输出：
false
public class test {
public static void main(String[] args) {
Cat c1=new Cat(1,2,3);
Cat c2=new Cat(1,2,3);
System.out.println(c1==c2);
System.out.println (c1.equals(c2));
}
}
class Cat {
int color;
int height,weight;
public Cat(int color,int height,int weight) {
this.color=color;
this.height=height;
this.weight=weight;
}
//重写方法
public boolean equals(Object obj) {
return true;
}//永远都相等，不好！就是写在这里需要注意一下重写要从JDK文档中COPY一下
}
//永远不等，比较的是地址
输出：
false
true
重写equals方法正确写法
public class test {
public static void main(String[] args) {
Cat c1=new Cat(1,2,3);
Cat c2=new Cat(1,2,3);
System.out.println(c1==c2);
System.out.println (c1.equals(c2));
}
}
class Cat {
int color;
int height,weight;
public Cat(int color,int height,int weight) {
this.color=color;
this.height=height;
this.weight=weight;
}
//重写方法
public boolean equals(Object obj) {
if(obj==null) return false;
else {
if(obj instanceof Cat) {//如果obj是猫对象的一个引用
Cat c=(Cat)obj;//obj强制转化为猫类型
if(c.color==this.color&&c.height==this.height&&c.weight==this.weight) {
return true;
}
}
}
return false;
}
}
输出：
false
true
public class test {
public static void main(String[] args) {
Cat c1=new Cat(1,2,3);
Cat c2=new Cat(1,2,3);
System.out.println(c1==c2);
System.out.println (c1.equals(c2));
String s1=new String("Hello");
String s2=new String("Hello");
System.out.println(s1==s2);
//java.lang包下的string重写了equals方法
//将此字符串与指定的对象比较。当且仅当该参数不为 null，并且是与此对象表示相同字符序列的 String 对象时，结果才为 true。
System.out.println(s1.equals(s2));//s2不为空，是true
}
}
class Cat {
int color;
int height,weight;
public Cat(int color,int height,int weight) {
this.color=color;
this.height=height;
this.weight=weight;
}
//重写方法
public boolean equals(Object obj) {
if(obj==null) return false;
else {
if(obj instanceof Cat) {//如果obj是猫对象的一个引用
Cat c=(Cat)obj;//obj强制转化为猫类型
if(c.color==this.color&&c.height==this.height&&c.weight==this.weight) {
return true;
}
}
}
return false;
}
}
输出：
false
true
false
true
equals
public boolean equals(Object obj)
- 指示其他某个对象是否与此对象“相等”。
equals方法在非空对象引用上实现相等关系：-
自反性：对于任何非空引用值
x，x.equals(x)都应返回true。 -
对称性：对于任何非空引用值
x和y，当且仅当y.equals(x)返回true时，x.equals(y)才应返回true。 -
传递性：对于任何非空引用值
x、y和z，如果x.equals(y)返回true，并且y.equals(z)返回true，那么x.equals(z)应返回true。 -
一致性：对于任何非空引用值
x和y，多次调用 x.equals(y) 始终返回true或始终返回false，前提是对象上equals比较中所用的信息没有被修改。 - 对于任何非空引用值
x，x.equals(null)都应返回false。
Object类的 equals 方法实现对象上差别可能性最大的相等关系；即，对于任何非空引用值x和y，当且仅当x和y引用同一个对象时，此方法才返回true（x == y具有值true）。注意：当此方法被重写时，通常有必要重写 hashCode 方法，以维护 hashCode 方法的常规协定，该协定声明相等对象必须具有相等的哈希码。
-
自反性：对于任何非空引用值
-
- 参数：
-
obj- 要与之比较的引用对象。 - 返回：
- 如果此对象与 obj 参数相同，则返回
true；否则返回false。 - 另请参见：
-
hashCode(),Hashtable
对象转型：
class Animal {
public String name;
Animal(String name) {
this.name=name;
}
}
class Cat extends Animal {
public String eyesColor;
Cat(String n,String c) {
super(n);
eyesColor=c;
}
}
class Dog extends Animal {
public String furcolor;
Dog(String n,String c) {
super(n);
furcolor=c;
}
}
public class test {
public static void main(String[] args) {
Animal a=new Animal("name");
Cat c=new Cat("name","blue");
Dog d=new Dog("dogname","black");
System.out.println(a instanceof Animal);//true
System.out.println(c instanceof Animal);//true
System.out.println(d instanceof Animal);//true
System.out.println(a instanceof Cat);//false
a=new Dog("bigyellow","yellow");;
System.out.println(a.name);//bigyellow
//System.out.println(a.furname);//error!
System.out.println(a instanceof Animal);//true
System.out.println(a instanceof Dog);//true
Dog d1=(Dog)a;//要加强制你转换符
System.out.println(d1.furcolor);//yellow
}
}
输出：
true
true
true
false
bigyellow
true
true
yellow
class Animal {
public String name;
Animal(String name) {
this.name=name;
}
}
class Cat extends Animal {
public String eyesColor;
Cat(String n,String c) {
super(n);
eyesColor=c;
}
}
class Dog extends Animal {
public String furcolor;
Dog(String n,String c) {
super(n);
furcolor=c;
}
}
public class test {
public static void main(String[] args) {
test testdemo=new test();
Animal a=new Animal("name");
Cat c=new Cat("catname","blue");
Dog d=new Dog("dogname","black");
testdemo.f(a);
testdemo.f(c);
testdemo.f(d);
}
public void f(Animal a) {
System.out.println("name:"+a.name);
if(a instanceof Cat) {
Cat cat=(Cat) a;
System.out.println(" "+cat.eyesColor+" eyes");
}
else if(a instanceof Dog) {
Dog dog =(Dog) a;
System.out.println(" "+dog.furcolor+" fur");
}
}
}输出：
name:name
name:catname
blue eyes
name:dogname
black fur
abstract class Animal {//有了抽象方法，这个类必须被声明为抽象类
private String name;
Animal(String name) {this.name = name;}
/*
public void enjoy(){
System.out.println("叫声......");
}
*/
//抽象类的方法没有写的必要
public abstract void enjoy();//只有;没有左右大括号定义，相当于C++里的纯虚函数
}
abstract class Cat extends Animal {
private String eyesColor;
Cat(String n,String c) {super(n); eyesColor = c;}
/*
public void enjoy() {
System.out.println("猫叫声......");
}
*/
//public abstract void enjoy();
}
class Dog extends Animal {
private String furColor;
Dog(String n,String c) {super(n); furColor = c;}
public void enjoy() {
System.out.println("狗叫声......");
}
}
class Bird extends Animal {
Bird() {
super("bird");
}
public void enjoy() {
System.out.println("鸟叫声......");
}
}
class Lady {
private String name;
private Animal pet;
Lady(String name,Animal pet) {
this.name = name; this.pet = pet;
}
public void myPetEnjoy(){pet.enjoy();}
}
public class test {
public static void main(String args[]){
//Cat c = new Cat("catname","blue");//抽象的类是残缺的类，NEW不出来
Dog d = new Dog("dogname","black");
Bird b = new Bird();
//Lady l1 = new Lady("l1",c);
Lady l2 = new Lady("l2",d);
Lady l3 = new Lady("l3",b);
//l1.myPetEnjoy();
l2.myPetEnjoy();
l3.myPetEnjoy();
}
}
输出：
狗叫声......
鸟叫声......
final关键字
public class test {
public static void main(String[] args) {
T t=new T();
System.out.println(t.i);
}
}
final class T {
final int i = 8;
public final void m() {
//j=9;
}
}
输出：
8
说白了，就是只可以读不可以写
接口：
interface Singer {
public void sing();
public void sleep();
}
interface Painter {
public void paint();
public void eat();
}
class Student implements Singer {
private String name;
Student(String name) {
this.name=name;
}
public void study() {
System.out.println("studying");
}
public String getName() {
return name;
}
public void sing() {
System.out.println("student is singing");
}
public void sleep() {
System.out.println("student is sleeping");
}
}
class Teacher implements Singer,Painter {
private String name;
public String getString() {
return name;
}
Teacher(String name) {this.name=name;}
public void teach() {
System.out.println("teacher is teaching");
}
public void sing() {
System.out.println("teacher is singing");
}
public void sleep() {
System.out.println("teacher is sleeping");
}
public void eat() {
System.out.println("teacher is eating");
}
@Override
public void paint() {
// TODO Auto-generated method stub
System.out.println("teacher is painting");
}
}
public class test {
public static void main(String[] args) {
Singer s1=new Student("tom");
s1.sing();
s1.sleep();
Teacher t1=new Teacher("jack");
t1.eat();
t1.sing();
t1.sleep();
t1.paint();
Painter p1=(Painter)t1;
p1.paint();
p1.eat();
}
}
输出：
student is singing
student is sleeping
teacher is eating
teacher is singing
teacher is sleeping
teacher is painting
teacher is painting
teacher is eating