解释器模式

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类图

代码

Expression

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interface Expression{
    boolean interpret(Context ctx);
}

Context

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class Context{

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

    public void assgin(Variable var, boolean value){
        map.put(var, value);
    }

    public Boolean lookup(Variable var){
        return map.get(var);
    }
}

TerminalExpression

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class Variable implements Expression{

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

}

class Constant implements Expression{

    private Boolean value;

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

    @Override
    public boolean interpret(Context ctx) {
        return value;
    }
}

NotTerminalExpression

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class And implements Expression{

    private Expression left, right;

    public And(Expression left, Expression right) {
        this.left = left;
        this.right = right;
    }

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

class Or implements Expression{

    private Expression left, right;

    public Or(Expression left, Expression right) {
        this.left = left;
        this.right = right;
    }

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

Client

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public class Main {
    public static void main(String args[]){

        Context ctx = new Context();

        Variable a = new Variable();
        Constant b = new Constant(true);
        ctx.assgin(a, false);

        Expression result = new And(new Or(a, b), a);

        System.out.println(result.interpret(ctx));
    }
}

总结

优点

新文法的实现较为便捷

缺点

执行效率较低，可利用的场景比较少

复杂文法比较难维护