Guava PreConditions

Preconditions类提供了一系列静态方法，用来检测方法或者构造函数在调用时参数是否合法，如果不合法，则抛出异常。Preconditions有三种形式：

1. 无报错信息，直接抛出异常
2. 存在一个额外的Object参数作为报错信息，方法参数不合法时，额外的Object参数作为异常的message返回
3. 存在一个额外的String模板参数作为报错信息，任意数量的Object填充String模板，方法参数不合法时，String模板填充后作为异常的message返回

• checkArgument

public void pintAgeNoErrorMessage(int age) {
    Preconditions.checkArgument(age > 0);
    System.out.println("age is: " + age);
}

public void pintAgeWithErrorMessage(int age) {
    String message = "Age can't be zero or less than zero.";
    Preconditions.checkArgument(age > 0, message);
    System.out.println("age is: " + age);
}

public void pintAgeWithTemplateErrorMessage(int age) {
    String message = "Age should be positive number, you supplied %s.";
    Preconditions.checkArgument(age > 0, message, age);
    System.out.println("age is: " + age);
}

@Test
public void checkArgumentTest() {
    int age = -18;
    assertThatThrownBy(() -> pintAgeNoErrorMessage(age))
            .isInstanceOf(IllegalArgumentException.class)
            .hasMessage(null).hasNoCause();

    String message = "Age can't be zero or less than zero.";
    assertThatThrownBy(() -> pintAgeWithErrorMessage(age))
            .isInstanceOf(IllegalArgumentException.class)
            .hasMessage(message).hasNoCause();

    String message1 = "Age should be positive number, you supplied %s.";
    assertThatThrownBy(() -> pintAgeWithTemplateErrorMessage(age))
            .isInstanceOf(IllegalArgumentException.class)
            .hasMessage(message1, age).hasNoCause();
}

• checkElementIndex

@Test
public void checkElementIndexTest() {
    int[] numbers = { 1, 2, 3, 4, 5 };
    String message = "Please check the bound of an array and retry";

    assertThatThrownBy(() ->
            Preconditions.checkElementIndex(6, numbers.length, message))
            .isInstanceOf(IndexOutOfBoundsException.class)
            .hasMessageStartingWith(message).hasNoCause();
}

• checkPositionIndex

@Test
public void checkPositionIndexTest() {
    int[] numbers = { 1, 2, 3, 4, 5 };
    String message = "Please check the bound of an array and retry";

    assertThatThrownBy(
            () -> Preconditions.checkPositionIndex(6, numbers.length, message))
            .isInstanceOf(IndexOutOfBoundsException.class)
            .hasMessageStartingWith(message).hasNoCause();
}

• checkNotNull

@Test
public void checkNotNullTest () {
    String nullObject = null;
    String message = "Please check the Object supplied, its null!";

    assertThatThrownBy(() -> Preconditions.checkNotNull(nullObject, message))
            .isInstanceOf(NullPointerException.class)
            .hasMessage(message).hasNoCause();
}

• checkState

@Test
public void checkStateTest() {
    int[] validStates = { -1, 0, 1 };
    int givenState = 10;
    String message = "You have entered an invalid state";

    assertThatThrownBy(
            () -> Preconditions.checkState(
                    Arrays.binarySearch(validStates, givenState) > 0, message))
            .isInstanceOf(IllegalStateException.class)
            .hasMessageStartingWith(message).hasNoCause();
}

以上就是Guava PreConditions的几种常用方法，仔细观察会发现，其实checkNotNull、checkState都可以通过checkArgument实现。在实际开发中，像注解@NotNull也可以实现类似的功能，且更加方便。

测试代码：码云 – 卓立 – Guava Preconditions

1. Google Guava Docs