设计模式（第二十二式：状态模式）

概念：
　　状态模式：Allow an object to alter its benhavior when its internal state changes. The object will appear to change its class. 当一个对象内在状态改变是允许改变行为，这个对象看起来像改变了其类型。

实现：
　　定义状态接口或抽象状态类

    public abstract class AbstractCourse {
        public AbstractCourse(String teacherName) {
            this.teacherName = teacherName;
        }

        private String teacherName;

        public abstract void study();

        public String getTeacherName() {
            return teacherName;
        }

        public void setTeacherName(String teacherName) {
            this.teacherName = teacherName;
        }
    }

　　状态具体实现

    public class BiologyClass extends AbstractCourse {

        public BiologyClass(String teacherName) {
            super(teacherName);
        }

        @Override
        public void study() {
            System.out.println(this.getTeacherName() + "正在上生物课");
        }
    }

    public class MathClass extends AbstractCourse {

        public MathClass(String teacherName) {
            super(teacherName);
        }

        @Override
        public void study() {
            System.out.println(this.getTeacherName() + "正在上数学课");
        }
    }

　　具体应用场景

    public class School {
        private AbstractCourse math = new MathClass("数学老师");
        private AbstractCourse biology = new BiologyClass("生物老师");

        private AbstractCourse now;

        public void study(){
            now.study();
        }

        public AbstractCourse getMath() {
            return math;
        }

        public void setMath(AbstractCourse math) {
            this.math = math;
        }

        public AbstractCourse getBiology() {
            return biology;
        }

        public void setBiology(AbstractCourse biology) {
            this.biology = biology;
        }

        public AbstractCourse getNow() {
            return now;
        }

        public void setNow(AbstractCourse now) {
            this.now = now;
        }
    }

测试及结果：

    public class StateTest {

        @Test
        public void statetest(){
            School school = new School();
            school.setNow(school.getMath());
            school.study();

            school.setNow(school.getBiology());
            school.study();
        }
    }

　　数学老师正在上数学课
　　生物老师正在上生物课
分析：
　　1.状态过多会导致子类过多。
　　2.封装性好，在不改变应用场景的情况下，实际上是改变了使用的不同类，但是对外外部而言是种改变了其中的属性（状态）。