结构
(1)抽象表达式(Expression)角色:声明一个所有的具体表达式角色都需要实现的抽象接口。这个接口主要是一个interpret()方法,称做解释操作。
(2)终结符表达式(Terminal Expression)角色:实现了抽象表达式角色所要求的接口,主要是一个interpret()方法;文法中的每一个终结符都有一个具体终结表达式与之相对应。比如有一个简单的公式R=R1+R2,在里面R1和R2就是终结符,对应的解析R1和R2的解释器就是终结符表达式。
(3)非终结符表达式(Nonterminal Expression)角色:文法中的每一条规则都需要一个具体的非终结符表达式,非终结符表达式一般是文法中的运算符或者其他关键字,比如公式R=R1+R2中,“+"就是非终结符,解析“+”的解释器就是一个非终结符表达式。
(4)环境(Context)角色:这个角色的任务一般是用来存放文法中各个终结符所对应的具体值,比如R=R1+R2,我们给R1赋值100,给R2赋值200。这些信息需要存放到环境角色中,很多情况下我们使用Map来充当环境角色就足够了。
为了说明解释器模式的实现办法,这里给出一个最简单的文法和对应的解释器模式的实现,这就是模拟Java语言中对布尔表达式进行操作和求值。
在这个语言中终结符是布尔变量,也就是常量true和false。非终结符表达式包含运算符and,or和not等布尔表达式。这个简单的文法如下:
Expression ::= Constant | Variable | Or | And | Not
And ::= Expression 'AND' Expression
Or ::= Expression 'OR' Expression
Not ::= 'NOT' Expression
Variable ::= 任何标识符
Constant ::= 'true' | 'false'
解释器模式的结构图如下所示:
package com.jiaozg.patterns.interpreter; public abstract class Expression { public abstract boolean interpret(Context ctx); public abstract boolean equals(Object obj); public abstract int hashCode(); public abstract String toString(); }
package com.jiaozg.patterns.interpreter; import java.util.HashMap; import java.util.Map; public class Context { private Map<Variable, Boolean> map = new HashMap<Variable, Boolean>(); public void assign(Variable var, boolean value) { map.put(var, Boolean.valueOf(value)); } public boolean lookup(Variable var) { Boolean value = map.get(var); if(value == null) { throw new IllegalArgumentException(); } return value.booleanValue(); } }
package com.jiaozg.patterns.interpreter; //一个Variable对象代表一个有名变量 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 boolean interpret(Context ctx) { return ctx.lookup(this); } @Override public String toString() { return name; } }
package com.jiaozg.patterns.interpreter; 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() + ")"; } }
package com.jiaozg.patterns.interpreter; 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 left.equals(((Or)obj).left) && 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() + ")"; } }
package com.jiaozg.patterns.interpreter; 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() + ")"; } }
package com.jiaozg.patterns.interpreter; //一个Constant对象代表一个布尔常量 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 Boolean.valueOf(value).toString(); } }
package com.jiaozg.patterns.interpreter; 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("c=" + c.interpret(ctx)); System.out.println("x=" + x.interpret(ctx)); System.out.println("y=" + y.interpret(ctx)); System.out.println(exp.toString() + "=" + exp.interpret(ctx)); } }
输出结果:
c=true
x=false
y=true
((true AND x) OR (y AND (Not x)))=true
参考:http://blog.csdn.net/m13666368773/article/details/7712110