How to implement a workflow using the state pattern in Java?

In Java, the state pattern can be used to implement a workflow. This pattern is a type of behavioral design pattern that allows objects to change their behavior when their internal state changes.

Firstly, we need to define the statuses of the workflow. Let’s assume there are three states: pending, in progress, and completed.

public interface WorkflowState {
    void handle(WorkflowContext context);
}

public class TodoState implements WorkflowState {
    @Override
    public void handle(WorkflowContext context) {
        System.out.println("当前状态：待办");
        // 处理待办事项
        // 设置下一状态
        context.setState(new InProgressState());
    }
}

public class InProgressState implements WorkflowState {
    @Override
    public void handle(WorkflowContext context) {
        System.out.println("当前状态：进行中");
        // 处理进行中事项
        // 设置下一状态
        context.setState(new CompletedState());
    }
}

public class CompletedState implements WorkflowState {
    @Override
    public void handle(WorkflowContext context) {
        System.out.println("当前状态：已完成");
        // 处理已完成事项
        // 设置下一状态
        context.setState(new TodoState());
    }
}

Next, we need to define a workflow context to store the current state and methods to handle the states.

public class WorkflowContext {
    private WorkflowState state;
    
    public WorkflowContext() {
        this.state = new TodoState();
    }
    
    public void handle() {
        state.handle(this);
    }
    
    public WorkflowState getState() {
        return state;
    }
    
    public void setState(WorkflowState state) {
        this.state = state;
    }
}

Finally, we can use the workflow context to handle the workflow.

public class Main {
    public static void main(String[] args) {
        WorkflowContext context = new WorkflowContext();
        
        context.handle(); // 打印：当前状态：待办
        context.handle(); // 打印：当前状态：进行中
        context.handle(); // 打印：当前状态：已完成
        context.handle(); // 打印：当前状态：待办
    }
}

In this way, we have implemented a simple workflow using the state pattern. Each time the workflow is processed, the workflow context will call the corresponding handling method based on the current state, and update the state to the next one.