编辑“︁Java解释器模式”︁（章节）

=== 案例：简单规则引擎 ===
假设我们需要实现一个简单的规则引擎，用于评估业务规则。例如：
* 规则1：如果用户是VIP且订单金额大于1000，则给予折扣。
* 规则2：如果商品类别是"电子产品"且库存小于10，则显示警告。

可以使用解释器模式来实现这样的规则系统：
<syntaxhighlight lang="java">
// 规则表达式接口
interface RuleExpression {
    boolean interpret(Map<String, Object> context);
}

// 终结符表达式：等于比较
class Equals implements RuleExpression {
    private String key;
    private Object value;

    public Equals(String key, Object value) {
        this.key = key;
        this.value = value;
    }

    @Override
    public boolean interpret(Map<String, Object> context) {
        return value.equals(context.get(key));
    }
}

// 非终结符表达式：AND逻辑
class And implements RuleExpression {
    private RuleExpression left;
    private RuleExpression right;

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

    @Override
    public boolean interpret(Map<String, Object> context) {
        return left.interpret(context) && right.interpret(context);
    }
}

// 使用示例
public class RuleEngineDemo {
    public static void main(String[] args) {
        // 定义上下文
        Map<String, Object> context = new HashMap<>();
        context.put("userType", "VIP");
        context.put("orderAmount", 1200);
        context.put("category", "Electronics");
        context.put("stock", 5);

        // 构建规则1：userType == "VIP" AND orderAmount > 1000
        RuleExpression rule1 = new And(
            new Equals("userType", "VIP"),
            new GreaterThan("orderAmount", 1000)
        );

        // 构建规则2：category == "Electronics" AND stock < 10
        RuleExpression rule2 = new And(
            new Equals("category", "Electronics"),
            new LessThan("stock", 10)
        );

        System.out.println("Rule1: " + rule1.interpret(context));  // true
        System.out.println("Rule2: " + rule2.interpret(context));  // true
    }
}
</syntaxhighlight>