一、默認構(gòu)造器

public class JavaA {
    public void hello(){
        System.out.println("my name is A ! ");
    }
    
    public static void main(String[] args) {
        JavaA obj=new JavaA();
        obj.hello();
    }
}

二、無參構(gòu)造器

public class JavaB {
    
    public JavaB (){
    
    }

    public void hello(){
        System.out.println("my name is javaB ! ");
    }
    
    public static void main(String[] args) {
        JavaB obj=new JavaB ();
        obj.hello();
    }
}

三、有參構(gòu)造器

public class JavaC {
    
    public String name="default";

    public JavaC(){
    
    }

    public JavaC(String name){
        this.name=name;
    }

    public void hello(){
        System.out.println("my name is  " + name);
    }
    
    public static void main(String[] args) {
        JavaC obj1=new JavaC ();
        obj1.hello();

        JavaC  obj2=new JavaC ("guoyasoft");
        obj2.hello();
    }
}

如果沒有自定義構(gòu)造器，則會默認自動生成一個無參構(gòu)造器
如果有自定義構(gòu)造器，則不再自動生成無參構(gòu)造器，如果要使用，需要自己定義一個無參構(gòu)造器

public class JavaC {
    
    public String name="default";

    public JavaC(String name){
        this.name=name;
    }

    public void hello(){
        System.out.println("my name is  " + name);
    }
    
    public static void main(String[] args) {
        JavaC obj1=new JavaC ();
        obj1.hello();

        JavaC  obj2=new JavaC ("guoyasoft");
        obj2.hello();
    }
}