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依赖倒置原则

• 定义：高层模块不应该依赖底层模块，二者都应该依赖其抽象
• 抽象不应该依赖细节；细节应该依赖抽象
• 针对接口编程，不要针对实现编程
• 优点：可以减少类之间的耦合性、提高系统稳定性，提高代码可读性和可维护性，可降低修改程序所造成的风险

public interface Course {
       void study();}

public class JavaCourse implements Course {
       @Override    public void study() {
           System.out.println("学习java课程");    }}

public class PythonCourse implements Course {
       @Override    public void study() {
           System.out.println("学习Python课程");    }}

public class SparkCourse implements Course {
       @Override    public void study() {
           System.out.println("学习Spark课程");    }}

public class Tom {
       private Course course;    public Tom(Course course) {
           this.course = course;    }    public Tom() {
       }    public void wantToStudy(Course course) {
           course.study();    }    public void wantToStudy() {
           course.study();    }    public void setCourse(Course course) {
           this.course = course;    }}

public class Test {
       public static void main(String[] args) {
           System.out.println("----------- v1 -------------");        Tom tom = new Tom();        tom.wantToStudy(new JavaCourse());        tom.wantToStudy(new PythonCourse());        tom.wantToStudy(new SparkCourse());        System.out.println("----------- v2 ----------");        Tom tom1 = new Tom(new JavaCourse());        tom1.wantToStudy();        System.out.println("----------- v3 ----------");        Tom tom2 = new Tom();        tom2.setCourse(new JavaCourse());        tom2.wantToStudy();        tom2.setCourse(new PythonCourse());        tom2.wantToStudy();    }}

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