Android中十进制减法的近似结果

Question

我在计算器应用程序中遇到了很多问题，但这两个是我的主要问题：

我的计算器在除减法之外的所有算术运算中都能正常工作。每当我执行.7-.5之类的操作时，它都会显示.199999 我需要使用删除单个数字而不是清除整个编辑文本。

package jarvis.calc;

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


import java.io.IOException;

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

float ValueOne, ValueTwo;
Boolean add=false, sub=false, div=false, multi=false ,deci = false;

@Override
protected void onCreate(Bundle savedInstanceState) {

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Button one = (Button) findViewById(R.id.one);
    Button two = (Button) findViewById(R.id.two);
    Button three = (Button) findViewById(R.id.three);
    Button four = (Button) findViewById(R.id.four);
    Button five = (Button) findViewById(R.id.five);
    Button six = (Button) findViewById(R.id.six);
    Button seven = (Button) findViewById(R.id.seven);
    Button eight = (Button) findViewById(R.id.eight);
    Button nine = (Button) findViewById(R.id.nine);
    Button zero = (Button) findViewById(R.id.zero);
    final Button decimal = (Button) findViewById(R.id.decimal);
    Button plus = (Button) findViewById(R.id.plus);
    Button minus = (Button) findViewById(R.id.minus);
    Button multiply = (Button) findViewById(R.id.multiply);
    Button divide = (Button) findViewById(R.id.divide);
    Button del = (Button) findViewById(R.id.del);
    Button eq = (Button) findViewById(R.id.eq);
    final EditText calculation = (EditText) findViewById(R.id.calculation);

    one.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View view) {
            calculation.setText(calculation.getText() + "1");
        }
    });

    two.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View view) {
            calculation.setText(calculation.getText() + "2");
        }
    });

    three.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View view) {
            calculation.setText(calculation.getText() + "3");
        }
    });

    four.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View view) {
            calculation.setText(calculation.getText() + "4");
        }
    });

    five.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View view) {
            calculation.setText(calculation.getText() + "5");
        }
    });

    six.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation.setText(calculation.getText() + "6");
        }
    });

    seven.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation.setText(calculation.getText() + "7");
        }
    });

    eight.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation.setText(calculation.getText() + "8");
        }
    });

    nine.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation.setText(calculation.getText() + "9");
        }
    });

    zero.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            calculation.setText(calculation.getText() + "0");
        }
    });

    decimal.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if(calculation.getText().toString().trim().length()== 0 && deci == false) {
                calculation.setText(calculation.getText() + ".");
                deci = true;
            }
            else if (calculation.getText().toString().trim().length() > 0 && deci == false){
                calculation.setText(calculation.getText() + ".");
                deci = true;
            }
        }
    });

    plus.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (calculation.getText().toString().trim().length() > 0){
                ValueOne = Float.parseFloat(calculation.getText() +"");
                deci=false;
                add = true;
                calculation.setText(null);
            }

            else if(calculation == null){
                calculation.setText("");
            }
        }
    });

    minus.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (calculation == null) {
                calculation.setText("");

            } else if (calculation.getText().toString().trim().length() > 0){
                ValueOne = Float.parseFloat(calculation.getText() + "");
                deci=false;
                sub = true;
                calculation.setText("");
            }
        }
    });

    multiply.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (calculation == null) {
                calculation.setText("");
            } else if (calculation.getText().toString().trim().length() > 0){
                ValueOne = Float.parseFloat(calculation.getText() + "");
                deci=false;
                multi = true;
                calculation.setText("");
            }
        }
    });

    divide.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (calculation == null) {
                calculation.setText("");
            } else if (calculation.getText().toString().trim().length() > 0){
                ValueOne = Float.parseFloat(calculation.getText() + "");
                deci=false;
                div = true;
                calculation.setText("");
            }
        }
    });

    eq.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            if (calculation.getText().toString().trim().length() > 0) {
            ValueTwo = Float.parseFloat(calculation.getText() + "");
            if (add == true) {
                calculation.setText(ValueOne + ValueTwo + "");
                add=false;

            }

            if(sub == true){
                calculation.setText(ValueOne - ValueTwo + "");
                sub=false;
            }

            if (multi == true){
                calculation.setText(ValueOne*ValueTwo + "");
                multi = false;
            }

            if (div == true){
                calculation.setText(ValueOne/ValueTwo + "");
                div = false;
            }}

            else if (calculation == null){
                calculation.setText("");
            }

        }


    });

    del.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            deci=false;
           calculation.setText("");

        }
    });

}


@Override
public void onClick(View view) {

}
}

`