达内课程-面向对象之构造方法和重载

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

<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">    <Button        android:id="@+id/button1"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="student()"        android:onClick="doClick"        />    <Button        android:id="@+id/button2"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="student(id,name)"        android:onClick="doClick"/>    <Button        android:id="@+id/button3"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="student(id,name,gender)"        android:onClick="doClick"/>    <Button        android:id="@+id/button4"        android:layout_width="wrap_content"        android:layout_height="wrap_content"        android:text="student(id,name,gender,age)"        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 {    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));    }