请给出一个能直接在Android Studio上运行的Android小项目

好的，以下是一个简单的可以直接在Android Studio上运行的Android小项目，它是一个简单的计算器应用程序：

1. 新建一个Android Studio项目，选择 Empty Activity 模板。

2. 打开 activity_main.xml 文件，并添加一个 TextView 和多个 Button：

<LinearLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="10dp">

    <TextView
        android:id="@+id/resultTextView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textSize="30sp"
        android:padding="10dp"
        android:textColor="@android:color/black"
        android:background="@android:color/white"
        android:gravity="right"/>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:padding="10dp">

        <Button
            android:id="@+id/button1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="1"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/button2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="2"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/button3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="3"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/buttonPlus"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="+"
            android:layout_weight="1"/>

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:padding="10dp">

        <Button
            android:id="@+id/button4"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="4"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/button5"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="5"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/button6"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="6"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/buttonMinus"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="-"
            android:layout_weight="1"/>

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:padding="10dp">

        <Button
            android:id="@+id/button7"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="7"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/button8"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="8"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/button9"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="9"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/buttonMultiply"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="*"
            android:layout_weight="1"/>

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:padding="10dp">

        <Button
            android:id="@+id/buttonClear"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="C"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/buttonZero"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="0"
            android:layout_weight="1"/>

        <Button
            android:id="@+id/buttonEqual"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="="
            android:layout_weight="1"/>

        <Button
            android:id="@+id/buttonDivide"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="/"
            android:layout_weight="1"/>

    </LinearLayout>

</LinearLayout>

3. 打开 MainActivity.java 文件，并添加以下代码：

public class MainActivity extends AppCompatActivity {

    private TextView resultTextView;
    private Button button1, button2, button3, button4, button5, button6, button7, button8, button9, button0, buttonClear, buttonEqual, buttonPlus, buttonMinus, buttonMultiply, buttonDivide;

    private String lastOperation = "";
    private boolean isLastOperationEquals = false;

    private double lastNumber = 0;

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

        resultTextView = findViewById(R.id.resultTextView);

        button0 = findViewById(R.id.buttonZero);
        button1 = findViewById(R.id.button1);
        button2 = findViewById(R.id.button2);
        button3 = findViewById(R.id.button3);
        button4 = findViewById(R.id.button4);
        button5 = findViewById(R.id.button5);
        button6 = findViewById(R.id.button6);
        button7 = findViewById(R.id.button7);
        button8 = findViewById(R.id.button8);
        button9 = findViewById(R.id.button9);
        buttonClear = findViewById(R.id.buttonClear);
        buttonEqual = findViewById(R.id.buttonEqual);
        buttonPlus = findViewById(R.id.buttonPlus);
        buttonMinus = findViewById(R.id.buttonMinus);
        buttonMultiply = findViewById(R.id.buttonMultiply);
        buttonDivide = findViewById(R.id.buttonDivide);

        button0.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked("0");
            }
        });

        button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked("1");
            }
        });

        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked("2");
            }
        });

        button3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked("3");
            }
        });

        button4.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked("4");
            }
        });

        button5.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked("5");
            }
        });

        button6.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked("6");
            }
        });

        button7.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked("7");
            }
        });

        button8.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked("8");
            }
        });

        button9.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                numberClicked("9");
            }
        });

        buttonPlus.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                operationClicked("+");
            }
        });

        buttonMinus.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                operationClicked("-");
            }
        });

        buttonMultiply.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                operationClicked("*");
            }
        });

        buttonDivide.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                operationClicked("/");
            }
        });

        buttonClear.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                clearClicked();
            }
        });

        buttonEqual.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                equalsClicked();
            }
        });

    }

    private void numberClicked(String number) {
        if (isLastOperationEquals) {
            resultTextView.setText("");
            isLastOperationEquals = false;
        }

        String currentResult = resultTextView.getText().toString();

        if (currentResult.equals("0")) {
            currentResult = "";
        }

        currentResult += number;

        resultTextView.setText(currentResult);
    }

    private void operationClicked(String operation) {
        if (isLastOperationEquals) {
            isLastOperationEquals = false;
        }

        String currentResult = resultTextView.getText().toString();

        if (currentResult.equals("")) {
            return;
        }

        double currentNumber = Double.parseDouble(currentResult);

        if (!lastOperation.equals("")) {
            switch (lastOperation) {
                case "+":
                    lastNumber += currentNumber;
                    break;
                case "-":
                    lastNumber -= currentNumber;
                    break;
                case "*":
                    lastNumber *= currentNumber;
                    break;
                case "/":
                    lastNumber /= currentNumber;
                    break;
            }

            resultTextView.setText(Double.toString(lastNumber));
        } else {
            lastNumber = currentNumber;
        }

        lastOperation = operation;
    }

    private void clearClicked() {
        lastNumber = 0;
        lastOperation = "";
        resultTextView.setText("");
    }

    private void equalsClicked() {
        String currentResult = resultTextView.getText().toString();

        if (currentResult.equals("")) {
            return;
        }

        double currentNumber = Double.parseDouble(currentResult);

        if (!lastOperation.equals("")) {
            switch (lastOperation) {
                case "+":
                    lastNumber += currentNumber;
                    break;
                case "-":
                    lastNumber -= currentNumber;
                    break;
                case "*":
                    lastNumber *= currentNumber;
                    break;
                case "/":
                    lastNumber /= currentNumber;
                    break;
            }

            resultTextView.setText(Double.toString(lastNumber));
        }

        isLastOperationEquals = true;
    }

}

4. 运行该应用程序，您将看到一个简单的计算器应用程序，您可以使用它来进行简单的数学运算。