错误：尝试在空对象引用上调用接口方法

Question

那里。我在android中使用eval（）函数时遇到问题。 它说尝试在空对象引用上调用接口方法'javax.script.Bindings javax.script.ScriptEngine.eval（java.lang.String）'。 我看过这个主题，似乎我有一些未初始化的东西。但是，我检查了我的代码，但找不到它。 请帮帮我:)。

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.TextView;

import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

public class MainActivity extends AppCompatActivity {

    private TextView tvScreen;
    private TextView tvHistory;
    private static ScriptEngine engine;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        tvScreen = (TextView) findViewById(R.id.screen);
        tvHistory = (TextView) findViewById(R.id.history);
        engine = new ScriptEngineManager().getEngineByName("JavaScript");
    }

    public void expressionEdit(View view) {
        if (tvScreen.getText().toString().equals("0")) {
            tvScreen.setText("");
        }
        switch (view.getId()) {
            case R.id.number1:
                tvScreen.append("1");
                break;
            case R.id.number2:
                tvScreen.append("2");
                break;
            case R.id.number3:
                tvScreen.append("3");
                break;
            case R.id.number4:
                tvScreen.append("4");
                break;
            case R.id.number5:
                tvScreen.append("5");
                break;
            case R.id.number6:
                tvScreen.append("6");
                break;
            case R.id.number7:
                tvScreen.append("7");
                break;
            case R.id.number8:
                tvScreen.append("8");
                break;
            case R.id.number9:
                tvScreen.append("9");
                break;
            case R.id.number0:
                tvScreen.append("0");
                break;
            case R.id.buttonDot:
                tvScreen.append(".");
                break;
            case R.id.buttonPlus:
                tvScreen.append("+");
                break;
            case R.id.buttonMinus:
                tvScreen.append("-");
                break;
            case R.id.buttonTime:
                tvScreen.append("*");
                break;
            case R.id.buttonDivide:
                tvScreen.append("/");
                break;
            case R.id.buttonLeftP:
                tvScreen.append("(");
                break;
            case R.id.buttonRightP:
                tvScreen.append(")");
                break;
            case R.id.number00:
                tvScreen.append("00");
                break;
            case R.id.buttonPercent:
                tvScreen.append("%");
                break;
            case R.id.buttonAC:
                tvScreen.setText("0");
                tvHistory.setText("");
                break;
            case R.id.delete:
                String temp = tvScreen.getText().toString();
                if (temp.length()>1){
                    tvScreen.setText(temp.substring(0,temp.length()-1));
                }
                if (temp.length()==1){
                    tvScreen.setText("0");
                }
                break;
        }
    }

    public void equalClicked(View view) {
        tvHistory.setText(tvScreen.getText().toString()+"=");
        String executableExp = expressionProcessor(tvScreen.getText().toString());
        try{
            String result = String.valueOf(engine.eval("1+1"));
            //String result = String.valueOf(engine.eval(executableExp));
            tvScreen.setText(result);
        } catch (Exception e) {
            Log.i("Error",e.getMessage());
            tvHistory.setText("Error");
        }

    }

    public String expressionProcessor(String originalExp){
        String executableExp;
        executableExp = originalExp.replaceAll("%","/100");
        return executableExp;
    }
}