关于java stack压栈出错

public class ReverseString {
Stack word;
Stack sentence;

    public void reverse(String sentence){
        for(int i=0; i<sentence.length(); i++){
            char n =sentence.charAt(i);

            if(n!=' '){
                word.push(n);
            }else{
                while(!word.empty()){
                    this.sentence.push(word.pop());
                }
                this.sentence.push(' ');
            }
        }
    }

    public void show(){
        while(!this.sentence.empty()){
            System.out.println(this.sentence.pop());
        }
    }
    public static void main(String[] args) {
        new ReverseString().reverse("the sky is blue");
    }

}

想不明白为何在执行word.push(n)时 会报 java.lang.NullPointerException

word和sentence你都 没有实例化，要用new