达内课程8-面向对象(二)

构造方法

1、新建实例时执行的特殊方法

new Soldier();
new FlashLight():
new Car():
new Dog();

新建实例的同时，调用一个构造方法

2、一个类必须有构造方法
3、如果不写构造方法，编辑器编译代码时，会默认添加一个构造方法，如下：

public class A(){
    //添加的默认无参空方法，没有返回类型
    public A(){
    }
}

4、构造方法重载（具有不同参数）
增加程序的灵活性

public class A(){
    public A(int i){
        ...
    }
    public A(double d){
        ...
    }
    public A(int i,double d){
        ...
    }
}

创建实例时，就不能写new A();
因为已经写了构造参数，运行时就不会再添加默认的构造方法

5、构造方法的作用，通常用来给成员变量赋值

举例1，给Dog类添加构造方法

public class Dog {
    public String name;//名字
    public int hungry;//饥饿度
    public int happy;//快乐度

    //构造方法
    public Dog(){
        Log.d("Dog","执行了构造方法");
    }

    //喂食
    public void feed(TextView textView){
        if(hungry==0){
            textView.setText(name+"吃饱了\n"+textView.getText());
            return;
        }
        hungry-=10;
        textView.setText("给"+name+"喂食,hungry:"+hungry+"\n"+textView.getText());
    }
......

那么在创建Dog类时，会执行Log输出

举例2，给Dog类添加构造方法

  public class Dog {
    public String name;//名字
    public int hungry;//饥饿度
    public int happy;//快乐度

    //构造方法
    public Dog(String name,int hungry,int happy){
        //如果局部变量与成员变量同名,必须用this.xx来引用成员变量,如果不同名，可以直接写
        this.name = name;
        this.happy = hungry;
        this.happy = happy;
    }
......

创建Dog对象的时候写法

dog = new Dog("蠢狗",50,50);

举例3，给Dog类添加构造方法

public class Dog {
    public String name;//名字
    public int hungry;//饥饿度
    public int happy;//快乐度

    //构造方法
    public Dog(){

    }
    public Dog(String name,int hungry,int happy){
        //如果局部变量与成员变量同名,必须用this.xx来引用成员变量
        this.name = name;
        this.happy = hungry;
        this.happy = happy;
    }
    ......

创建Dog对象的时候写法有两种

dog = new Dog("蠢狗",50,50);

dog = new Dog();

练习

Student类

public class Student {
    public int id;
    public String name;
    public String gender;
    public int age;

    //添加构造方法
    public Student(){

    }
    public Student(int id,String name){
        //减少代码重复,调用3个参数方法,没有的参数gender可以传默认值null
        /*this.id = id;
 this.name = name;*/
        this(id,name,null);
    }
    public Student(int id,String name,String gender){
        //减少代码重复,调用3个参数方法,没有的参数age可以传默认值0
        /* this.id = id;
 this.name = name;
 this.gender = gender;*/
        this(id,name,gender,0);
    }
    public Student(int id,String name,String gender,int age){
        this.id = id;
        this.name = name;
        this.gender = gender;
        this.age = age;
    }

    public String toString(){
        return "\n姓名："+name+"\n性别"+gender+"\n年龄"+age+"\nid"+id;
    }
}

xml

"http://schemas.android.com/apk/res/android"
              android:layout_hljs-string">"match_parent"
              android:layout_hljs-string">"match_parent"
              android:columnCount="4"
              android:orientation="vertical"
              android:rowCount="6">


            android:id="@+id/button1"
        android:layout_hljs-string">"wrap_content"
        android:layout_hljs-string">"wrap_content"
        android:text="student()"
        android:OnClick="doClick"
        />

            android:id="@+id/button2"
        android:layout_hljs-string">"wrap_content"
        android:layout_hljs-string">"wrap_content"
        android:text="student(id,name)"
        android:OnClick="doClick"/>

            android:id="@+id/button3"
        android:layout_hljs-string">"wrap_content"
        android:layout_hljs-string">"wrap_content"
        android:text="student(id,name,gender)"
        android:OnClick="doClick"/>

            android:id="@+id/button4"
        android:layout_hljs-string">"wrap_content"
        android:layout_hljs-string">"wrap_content"
        android:text="student(id,name,gender,age)"
        android:OnClick="doClick"/>

            android:id="@+id/textView"
        android:layout_hljs-string">"match_parent"
        android:layout_hljs-string">"wrap_content"
        />

MainActivity

public class MainActivity extends AppCompatActivity {
    Dog dog;

    Button create;
    Button button1;
    Button button2;
    Button button3;
    TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        create = (Button)findViewById(R.id.button1);
        button1 = (Button)findViewById(R.id.button2);
        button2 = (Button)findViewById(R.id.button3);
        button3 = (Button)findViewById(R.id.button4);
        textView = (TextView)findViewById(R.id.textView);
    }

    public void doClick(View view) {
        switch (view.getId()) {
            case R.id.button1:
                f1();
                break;
            case R.id.button2:
                f2();
                break;
            case R.id.button3:
                f3();
                break;
            case R.id.button4:
                f4();
                break;
        }
    }

    private void f1(){
        Student s = new Student();
        textView.setText(s.toString());
    }

    private void f2(){
        Student s = new Student(9527,"张三");
        textView.setText(s.toString());
    }
    private void f3(){
        Student s = new Student(9528,"lili","女");
        textView.setText(s.toString());
    }
    private void f4(){
        Student s = new Student(9529,"苏菲","女",22);
        textView.setText(s.toString());
    }
}

this

1、引用当前对象成员
this保存当前对象的内存地址
用this可以访问当前对象内存空间，访问当前对象成员
有同名局部变量时，必须使用this.xx访问成员变量
2、构造方法间调用
this();
从一个构造方法调用另一个重载的构造方法，目的：减少代码重复
一般从参数少的方法调用参数多的方法
必须是首行代码

正确

    public Student(int id,String name){
        this(id,name,null);
    }

错误

    public Student(int id,String name){
        System.out.println("xx");
        this(id,name,null);
    }

练习:求坐标点距离

Point类

public class Point {
     int x;
     int y;

    public Point(int x,int y){
        this.x = x;
        this.y = y;
    }

    public String tostring(){
        return "("+x+","+y+")";
    }

    public double distance(){
        return Math.sqrt(x*x+y*y);
    }
}

XML

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:columnCount="4"
 android:orientation="vertical"
 android:rowCount="6">

    <EditText
 android:id="@+id/xEditText"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:hint="x"/>
    <EditText
 android:id="@+id/yEditText"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 android:hint="y"/>
    <Button
 android:id="@+id/button1"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:text="student()"
 android:onClick="doClick"
 />

    <TextView
 android:id="@+id/textView"
 android:layout_width="match_parent"
 android:layout_height="wrap_content"
 />


LinearLayout>

MainActivity

public class MainActivity extends AppCompatActivity {
    Point point;

    EditText xEditText;
    EditText yEditText;
    Button button1;
    TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        xEditText = (EditText) findViewById(R.id.xEditText);
        yEditText = (EditText) findViewById(R.id.yEditText);
        button1 = (Button) findViewById(R.id.button1);
        textView = (TextView) findViewById(R.id.textView);
    }

    public void doClick(View view) {
        switch (view.getId()) {
            case R.id.button1:
                f1();
                break;
        }
    }

    private void f1() {
        String x = xEditText.getText().toString();
        String y = yEditText.getText().toString();
        Point point = new Point(Integer.parseInt(x),Integer.parseInt(y));
        textView.setText(point.tostring()+"\n"+point.distance());
    }
}

方法重载Overload

同名不同参

练习:求两点之间距离

Point类

public class Point {
     int x;
     int y;

    public Point(int x,int y){
        this.x = x;
        this.y = y;
    }

    public String tostring(){
        return "("+x+","+y+")";
    }

    public double distance(){
        return Math.sqrt(x*x+y*y);
    }

    public double distance(Point p){
        int dx = x-p.x;
        int dy=y-p.y;
        return Math.sqrt(dx*dx+dy*dy);
    }
}

xml

不变

MainActivity

仅修改f1方法

    private void f1() {
        int x = Integer.parseInt(xEditText.getText().toString());
        int y = Integer.parseInt(yEditText.getText().toString());
        Point point1 = new Point(x,y);
        textView.setText(point1.tostring()+"距远点距离"+point1.distance()+"\n");

        Point point2 = new Point(new Random().nextInt(10),new Random().nextInt(10));
        textView.append(point1.tostring()+"距离"+point2.tostring()+"\n"+point1.distance(point2));
    }