解决的问题

一个Excel表格，你修改一个地方的数值（DataSource），相关的图表(Chart)和另一个表格(SpreadSheet)也会同步发生变化。

代码

Observer:

package com.cong.designpattern.observer;

public interface Observer {
    public void update(int newValue);
}

DataSource:

package com.cong.designpattern.observer;

import java.util.ArrayList;
import java.util.List;

public class DataSource {
    private int value;
    private List<Observer> observers;
    public DataSource() {
        this.value = 0;
        this.observers = new ArrayList<>();
    }
    public int getValue() {
        return value;
    }
    public void setValue(int value) {
        this.value = value;
        this.notifyObservers(value);
    }
    public void addObserver(Observer observer) {
        this.observers.add(observer);
    }
    public void removeObserver(Observer observer) {
        this.observers.remove(observer);
    }
    private void notifyObservers(int value) {
        for (Observer observer : this.observers) {
            observer.update(value);
        }
    }
}

Chart:

package com.cong.designpattern.observer;

public class Chart implements Observer {
    @Override
    public void update(int newValue) {
        System.out.println("Chart did updated with " + newValue);
    }
}

SpreadSheet:

package com.cong.designpattern.observer;

public class SpreadSheet implements Observer {
    @Override
    public void update(int newValue) {
        System.out.println("Spread sheet did updated with " + newValue);
    }
}

Test code:

DataSource dataSource = new DataSource();
SpreadSheet spreadSheet = new SpreadSheet();
Chart chart = new Chart();
dataSource.addObserver(spreadSheet);
dataSource.addObserver(chart);
dataSource.setValue(1);

UML

Observer Pattern UML