設計模式之---解釋器模式

解釋器模式是個不怎麼常用的設計模式。我們先來舉一個簡單的例子說明它。

a+b-c這麼一個表達式，+,-都是非終結符表達式，a,b,c都是終結符表達式。非終結表達式一般是文法中的運算符或者關鍵字，而終結表達式一般是文法中的運算單元。

package interpreter;

import java.util.HashMap;
import java.util.Map;

public class Context {
	private final Map<String, Integer> valueMap = new HashMap<String, Integer>();

    public void addValue(final String key, final int value) {
        valueMap.put(key, Integer.valueOf(value));
    }

    public int getValue(final String key) {
        return valueMap.get(key).intValue();
    }
}

package interpreter;

public abstract class AbstractExpression {
	public abstract int interpreter(Context context);
}

package interpreter;

public class AddNonterminalExpression extends AbstractExpression {
    private final AbstractExpression left;
    private final AbstractExpression right;

    public AddNonterminalExpression(final AbstractExpression left, final AbstractExpression right) {
        this.left = left;
        this.right = right;
    }

    @Override
    public int interpreter(final Context context) {
        return this.left.interpreter(context) + this.right.interpreter(context);
    }

}

package interpreter;

public class SubtractNonterminalExpression extends AbstractExpression {
    private final AbstractExpression left;
    private final AbstractExpression right;

    public SubtractNonterminalExpression(final AbstractExpression left, final AbstractExpression right) {
        this.left = left;
        this.right = right;
    }

    @Override
    public int interpreter(final Context context) {
        return this.left.interpreter(context) - this.right.interpreter(context);
    }
}

package interpreter;

public class TerminalExpression extends AbstractExpression {
    private final int i;

    public TerminalExpression(final int i) {
        this.i = i;
    }

    @Override
    public int interpreter(final Context context) {
        return this.i;
    }

}

package interpreter;

import flyweight.Flyweight;
import flyweight.FlyweightFactory;
import flyweight.UnsharedFlyweight;

public class Client {
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		//a+b-c
		final Context context = new Context();
        context.addValue("a", 7);
        context.addValue("b", 8);
        context.addValue("c", 2);
        final AddNonterminalExpression addValue = new AddNonterminalExpression
        		(new TerminalExpression(context.getValue("a")),  new TerminalExpression(context.getValue("b")));
        final SubtractNonterminalExpression subtractValue = new SubtractNonterminalExpression
        		(addValue,  new TerminalExpression(context.getValue("c")));
        System.out.println(subtractValue.interpreter(context));
	}
}

解釋器模式的優缺點：

解釋器模式的應用場景和注意事項：