如何在Java中分别创建一个加法类（Addition）、减法类（Subtraction）、乘法类（Multiplication）和除法类（Division），并为每个类设计对应的单元测试用例来验证其功能是否正确？

在Java中，为了实现这种结构，我们可以创建四个单独的类，每个类代表一种数学运算。这里我将展示如何创建和测试每个类：

1. **Addition类**:

public class Addition {
    public int add(int a, int b) {
        return a + b;
    }
}

**单元测试**:

@Test
public void testAddition() {
    Addition addition = new Addition();
    assertEquals(5, addition.add(2, 3));
    assertEquals(0, addition.add(0, 0));
    assertNotEquals(-1, addition.add(1, -1)); // 验证负数加法
}

2. **Subtraction类**:

public class Subtraction {
    public int subtract(int a, int b) {
        return a - b;
    }
}

**单元测试**:

@Test
public void testSubtraction() {
    Subtraction subtraction = new Subtraction();
    assertEquals(1, subtraction.subtract(3, 2));
    assertEquals(0, subtraction.subtract(0, 0));
    assertNotEquals(5, subtraction.subtract(2, 3)); // 验证正负数减法
}

3. **Multiplication类**:

public class Multiplication {
    public int multiply(int a, int b) {
        return a * b;
    }
}

**单元测试**:

@Test
public void testMultiplication() {
    Multiplication multiplication = new Multiplication();
    assertEquals(8, multiplication.multiply(2, 4));
    assertEquals(0, multiplication.multiply(0, 5));
    assertNotEquals(-10, multiplication.multiply(2, -5)); // 验证负数乘法
}

4. **Division类** (注意这里是Java 7及以后版本，因为Java 6及更早版本没有`BigDecimal`用于精确除法):

import java.math.BigDecimal;

public class Division {
    public BigDecimal divide(int a, int b) {
        return BigDecimal.valueOf(a).divide(BigDecimal.valueOf(b));
    }
}

**单元测试**:

@Test
public void testDivision() {
    Division division = new Division();
    assertEquals(new BigDecimal("2"), division.divide(4, 2));
    assertEquals(BigDecimal.ZERO, division.divide(0, 5));
    try {
        division.divide(4, 0);
        fail("Should throw an exception when dividing by zero");
    } catch (ArithmeticException expected) {}
}