Java代碼不會？這個問題可能很多人都遇到過，因為java語言本身就是一個開源的語言，所以很多人都覺得自己學不會，其實只要掌握了基本的知識，就可以輕松上手了。下面是幾個簡單的Java代碼演練，涵蓋了變量、條件語句、循環語句等核心概念，希望對大家有所幫助。如果你想學習java，可以參考一下這篇文章。

1. 變量和基本輸出

```java

public class HelloWorld {

    public static void main(String[] args) {

        // 定義字符串和整數變量

        String name = "Alice";

        int age = 25;

        // 輸出變量到控制臺

        System.out.println("Hello, " + name + "!");

        System.out.println("You are " + age + " years old.");

    }

}

```

輸出結果：

```

Hello, Alice!

You are 25 years old.

```

2. 條件語句

```java

public class Grade {

    public static void main(String[] args) {

        // 定義變量作為成績

        int score = 85;

        // 根據成績輸出不同的等級

        if (score >= 90) {

            System.out.println("A");

        } else if (score >= 80) {

            System.out.println("B");

        } else if (score >= 70) {

            System.out.println("C");

        } else if (score >= 60) {

            System.out.println("D");

        } else {

            System.out.println("F");

        }

    }

}

```

輸出結果：

```

B

```

3. 循環語句

```java

public class Count {

    public static void main(String[] args) {

        // 從1到10累加

        int sum = 0;

        for (int i = 1; i <= 10; i++) {

            sum += i;

        }

        // 輸出累加結果

        System.out.println("1 + 2 + ... + 10 = " + sum);

    }

}

```

輸出結果：

```

1 + 2 + ... + 10 = 55

```

4. 數組和循環

```java

public class ArraySum {

    public static void main(String[] args) {

        // 定義數組并初始化

        int[] arr = {1, 2, 3, 4, 5};

        // 遍歷數組并求和

        int sum = 0;

        for (int i = 0; i < arr.length; i++) {

            sum += arr[i];

        }

        // 輸出數組元素和

        System.out.println("The sum of array elements is " + sum);

    }

}

```

輸出結果：

```

The sum of array elements is 15

```

5. 方法調用

```java

public class Calculator {

    public static void main(String[] args) {

        // 調用add方法

        int num1 = 10;

        int num2 = 5;

        int sum = add(num1, num2);

        // 輸出結果

        System.out.println(num1 + " + " + num2 + " = " + sum);

    }

    // 定義add方法

    public static int add(int a, int b) {

        return a + b;

    }

}

```

輸出結果：

```

10 + 5 = 15

```

6. 類和對象

```java

public class Rectangle {

    // 定義長和寬變量

    int length;

    int width;

    // 定義計算面積方法

    public int getArea() {

        return length * width;

    }

    public static void main(String[] args) {

        // 創建Rectangle對象并設置長和寬

        Rectangle rect = new Rectangle();

        rect.length = 5;

        rect.width = 3;

        // 輸出面積

        System.out.println("The area of rectangle is " + rect.getArea());

    }

}

```

輸出結果：

```

The area of rectangle is 15

```

7. 繼承和多態

```java

// 父類

class Animal {

    public void sound() {

        System.out.println("The animal makes a sound");

    }

}

// 子類

class Dog extends Animal {

    public void sound() {

        System.out.println("The dog says woof");

    }

}

// 子類

class Cat extends Animal {

    public void sound() {

        System.out.println("The cat says meow");

    }

}

// 測試類

public class AnimalSound {

    public static void main(String[] args) {

        // 創建Animal、Dog、Cat對象并調用sound方法

        Animal animal = new Animal();

        Dog dog = new Dog();

        Cat cat = new Cat();

        animal.sound(); // 輸出 "The animal makes a sound"

        dog.sound();    // 輸出 "The dog says woof"

        cat.sound();    // 輸出 "The cat says meow"

        // 多態：Animal類型的變量可以引用Dog、Cat對象并調用sound方法

        Animal ani1 = new Dog();

        Animal ani2 = new Cat();

        ani1.sound();   // 輸出 "The dog says woof"

        ani2.sound();   // 輸出 "The cat says meow"

    }

}

```

輸出結果：

```

The animal makes a sound

The dog says woof

The cat says meow

The dog says woof

The cat says meow

```

8. 接口和實現

```java

// 接口

interface Shape {

    public void draw();

    public double getArea();

}

// 實現類

class Rectangle implements Shape {

    private int length;

    private int width;

    public Rectangle(int length, int width) {

        this.length = length;

        this.width = width;

    }

    public void draw() {

        System.out.println("Drawing a rectangle");

    }

    public double getArea() {

        return length * width;

    }

}

// 實現類

class Circle implements Shape {

    private int radius;

    public Circle(int radius) {

        this.radius = radius;

    }

    public void draw() {

        System.out.println("Drawing a circle");

    }

    public double getArea() {

        return Math.PI * radius * radius;

    }

}

// 測試類

public class ShapeTest {

    public static void main(String[] args) {

        // 創建Rectangle、Circle對象并調用draw、getArea方法

        Rectangle rect = new Rectangle(5, 3);

        Circle circle = new Circle(2);

        rect.draw();    // 輸出 "Drawing a rectangle"

        System.out.println("The area of the rectangle is " + rect.getArea());  // 輸出 "The area of the rectangle is 15.0"

        circle.draw();  // 輸出 "Drawing a circle"

        System.out.println("The area of the circle is " + circle.getArea());    // 輸出 "The area of the circle is 12.566370614359172"

    }

}

```

輸出結果：

```

Drawing a rectangle

The area of the rectangle is 15.0

Drawing a circle

The area of the circle is 12.566370614359172

```

9. 異常處理

```java

import java.util.Scanner;

public class Divide {

    public static void main(String[] args) {

        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter the dividend: ");

        int dividend = scanner.nextInt();

        System.out.print("Enter the divisor: ");

        int divisor = scanner.nextInt();

        try {

            int quotient = dividend / divisor;

            System.out.println(dividend + " / " + divisor + " = " + quotient);

        } catch (ArithmeticException e) {

            System.out.println("Error: division by zero!");

        } finally {

            scanner.close();

        }

    }

}

```

輸出結果：

```

Enter the dividend: 10

Enter the divisor: 2

10 / 2 = 5

```

或者

```

Enter the dividend: 10

Enter the divisor: 0

Error: division by zero!

```

10. 文件讀寫

```java

import java.io.File;

import java.io.FileWriter;

import java.io.IOException;

import java.util.Scanner;

public class FileInOut {

    public static void main(String[] args) {

        // 寫入文件

        try {

            // 創建或打開文件

            File file = new File("output.txt");

            FileWriter writer = new FileWriter(file);

            // 寫入字符串

            writer.write("Hello world!\n");

            writer.write("This is a test file.\n");

            // 關閉文件

            writer.close();

        } catch (IOException e) {

            System.out.println("Error writing file!");

        }

        // 讀取文件

        try {

            // 打開文件

            File file = new File("output.txt");

            Scanner scanner = new Scanner(file);

            // 逐行讀取并輸出到控制臺

            while (scanner.hasNextLine()) {

                System.out.println(scanner.nextLine());

            }

            // 關閉文件

            scanner.close();

        } catch (IOException e) {

            System.out.println("Error reading file!");

        }

    }

}

```

輸出結果：

```

Hello world!

This is a test file.

```