设计模式学习笔记二十：解释器模式

动机

解释器（INTERPRETER）模式 ，属于类行为型模式。给定一个语言，定义它的文法的一种表示，并定义一个解释器，这个解释器使用该表示来解释语言中的句子。（摘自《GoF设计模式》）

个人见解

自己创造编程语言或者DSL的时候能用到，除此之外，个人认为基本没有什么场景。存在的目的就是为了个团圆，况且编译原理已经还给老师了。
再有就是这个跟状态机的调用流程很像。

UML结构图：

代码实现

注：代码内容实现来自http://www.cnblogs.com/java-my-life/archive/2012/06/19/2552617.html

---

1.Expression，即AbstractExpression

public abstract class Expression {
    /**
     * 以环境为准，本方法解释给定的任何一个表达式
     */
    public abstract boolean interpret(Context ctx);
    /**
     * 检验两个表达式在结构上是否相同
     */
    public abstract boolean equals(Object obj);
    /**
     * 返回表达式的hash code
     */
    public abstract int hashCode();
    /**
     * 将表达式转换成字符串
     */
    public abstract String toString();
}

2.Constant，即TerminalExpression

public class Constant extends Expression{

    private boolean value;

    public Constant(boolean value){
        this.value = value;
    }

    @Override
    public boolean equals(Object obj) {

        if(obj != null && obj instanceof Constant){
            return this.value == ((Constant)obj).value;
        }
        return false;
    }

    @Override
    public int hashCode() {
        return this.toString().hashCode();
    }

    @Override
    public boolean interpret(Context ctx) {

        return value;
    }

    @Override
    public String toString() {
        return new Boolean(value).toString();
    }

}

3.Variable

public class Variable extends Expression {

    private String name;

    public Variable(String name){
        this.name = name;
    }
    @Override
    public boolean equals(Object obj) {

        if(obj != null && obj instanceof Variable)
        {
            return this.name.equals(
                    ((Variable)obj).name);
        }
        return false;
    }

    @Override
    public int hashCode() {
        return this.toString().hashCode();
    }

    @Override
    public String toString() {
        return name;
    }

    @Override
    public boolean interpret(Context ctx) {
        return ctx.lookup(this);
    }

}

4.And Or Not

public class And extends Expression {

    private Expression left,right;

    public And(Expression left , Expression right){
        this.left = left;
        this.right = right;
    }
    @Override
    public boolean equals(Object obj) {
        if(obj != null && obj instanceof And)
        {
            return left.equals(((And)obj).left) &&
                right.equals(((And)obj).right);
        }
        return false;
    }

    @Override
    public int hashCode() {
        return this.toString().hashCode();
    }

    @Override
    public boolean interpret(Context ctx) {
        return left.interpret(ctx) && right.interpret(ctx);
    }

    @Override
    public String toString() {
        return "(" + left.toString() + " AND " + right.toString() + ")";
    }

}

public class Or extends Expression {
    private Expression left,right;

    public Or(Expression left , Expression right){
        this.left = left;
        this.right = right;
    }
    @Override
    public boolean equals(Object obj) {
        if(obj != null && obj instanceof Or)
        {
            return this.left.equals(((Or)obj).left) && this.right.equals(((Or)obj).right);
        }
        return false;
    }

    @Override
    public int hashCode() {
        return this.toString().hashCode();
    }

    @Override
    public boolean interpret(Context ctx) {
        return left.interpret(ctx) || right.interpret(ctx);
    }

    @Override
    public String toString() {
        return "(" + left.toString() + " OR " + right.toString() + ")";
    }

}

public class Not extends Expression {

    private Expression exp;

    public Not(Expression exp){
        this.exp = exp;
    }
    @Override
    public boolean equals(Object obj) {
        if(obj != null && obj instanceof Not)
        {
            return exp.equals(
                    ((Not)obj).exp);
        }
        return false;
    }

    @Override
    public int hashCode() {
        return this.toString().hashCode();
    }

    @Override
    public boolean interpret(Context ctx) {
        return !exp.interpret(ctx);
    }

    @Override
    public String toString() {
        return "(Not " + exp.toString() + ")";
    }

}

5.Context

public class Context {

    private Map<Variable,Boolean> map = new HashMap<Variable,Boolean>();

    public void assign(Variable var , boolean value){
        map.put(var, new Boolean(value));
    }

    public boolean lookup(Variable var) throws IllegalArgumentException{
        Boolean value = map.get(var);
        if(value == null){
            throw new IllegalArgumentException();
        }
        return value.booleanValue();
    }
}

6.TestDriver

public class Client {

    public static void main(String[] args) {
        Context ctx = new Context();
        Variable x = new Variable("x");
        Variable y = new Variable("y");
        Constant c = new Constant(true);
        ctx.assign(x, false);
        ctx.assign(y, true);

        Expression exp = new Or(new And(c,x) , new And(y,new Not(x)));
        System.out.println("x=" + x.interpret(ctx));
        System.out.println("y=" + y.interpret(ctx));
        System.out.println(exp.toString() + "=" + exp.interpret(ctx));
    }

}