Assertions

JUnit提供了许多重载的断言方法，这些方法均可以通过"import static org.junit.Assert.*"导入。方法的参数顺序一般都是([失败时打印的字符串消息]，期望值，实际值)。

特别要提到的一种断言是assertThat，它的参数是([失败时打印的字符串消息]，实际值，Matcher对象)，参数顺序和其他的断言方法正好相反。同时由于生成Matcher对象需要用到”org.hamcrest.CoreMatchers.*"里面的方法，所以使用assertThat的时候需要额外导入hamcrest-core.jar和hamcrest-library.jar （下载地址：http://search.maven.org/#search|ga|1|g%3Aorg.hamcrest）。关于AssertThat的详细介绍参见 ”Matchers and assertthat"。

Hamcrest是什么呢？它的官网是这样说的：amcrest is a library of matchers, which can be combined in to create flexible expressions of intent in tests。按我的理解就是Hamcrest是一个Library，它提供了一套匹配符Matcher，这些匹配符可读性高而且灵活，所以JUnit4引入了Hamcrest框架。

Hamcrest官网：http://hamcrest.org/。

Hamcrest主页：http://code.google.com/p/hamcrest/wiki/Tutorial

Hamcrest整个包在GitHub的下载地址：https://github.com/hamcrest/JavaHamcrest

Method Summary
static void	assertArrayEquals(boolean[] expecteds, boolean[] actuals)           Asserts that two boolean arrays are equal.
static void	assertArrayEquals(byte[] expecteds, byte[] actuals)           Asserts that two byte arrays are equal.
static void	assertArrayEquals(char[] expecteds, char[] actuals)           Asserts that two char arrays are equal.
static void	assertArrayEquals(double[] expecteds, double[] actuals, double delta)           Asserts that two double arrays are equal.
static void	assertArrayEquals(float[] expecteds, float[] actuals, float delta)           Asserts that two float arrays are equal.
static void	assertArrayEquals(int[] expecteds, int[] actuals)           Asserts that two int arrays are equal.
static void	assertArrayEquals(long[] expecteds, long[] actuals)           Asserts that two long arrays are equal.
static void	assertArrayEquals(Object[] expecteds,Object[] actuals)           Asserts that two object arrays are equal.
static void	assertArrayEquals(short[] expecteds, short[] actuals)           Asserts that two short arrays are equal.
static void	assertArrayEquals(String message, boolean[] expecteds, boolean[] actuals)           Asserts that two boolean arrays are equal.
static void	assertArrayEquals(String message, byte[] expecteds, byte[] actuals)           Asserts that two byte arrays are equal.
static void	assertArrayEquals(String message, char[] expecteds, char[] actuals)           Asserts that two char arrays are equal.
static void	assertArrayEquals(String message, double[] expecteds, double[] actuals, double delta)           Asserts that two double arrays are equal.
static void	assertArrayEquals(String message, float[] expecteds, float[] actuals, float delta)           Asserts that two float arrays are equal.
static void	assertArrayEquals(String message, int[] expecteds, int[] actuals)           Asserts that two int arrays are equal.
static void	assertArrayEquals(String message, long[] expecteds, long[] actuals)           Asserts that two long arrays are equal.
static void	assertArrayEquals(String message,Object[] expecteds,Object[] actuals)           Asserts that two object arrays are equal.
static void	assertArrayEquals(String message, short[] expecteds, short[] actuals)           Asserts that two short arrays are equal.
static void	assertEquals(double expected, double actual)           Deprecated. Use assertEquals(double expected, double actual, double delta) instead
static void	assertEquals(double expected, double actual, double delta)           Asserts that two doubles are equal to within a positive delta.
static void	assertEquals(float expected, float actual, float delta)           Asserts that two floats are equal to within a positive delta.
static void	assertEquals(long expected, long actual)           Asserts that two longs are equal.
static void	assertEquals(Object[] expecteds,Object[] actuals)           Deprecated. use assertArrayEquals
static void	assertEquals(Object expected,Object actual)           Asserts that two objects are equal.
static void	assertEquals(String message, double expected, double actual)           Deprecated. Use assertEquals(String message, double expected, double actual, double delta) instead
static void	assertEquals(String message, double expected, double actual, double delta)           Asserts that two doubles are equal to within a positive delta.
static void	assertEquals(String message, float expected, float actual, float delta)           Asserts that two floats are equal to within a positive delta.
static void	assertEquals(String message, long expected, long actual)           Asserts that two longs are equal.
static void	assertEquals(String message,Object[] expecteds,Object[] actuals)           Deprecated. use assertArrayEquals
static void	assertEquals(String message,Object expected,Object actual)           Asserts that two objects are equal.
static void	assertFalse(boolean condition)           Asserts that a condition is false.
static void	assertFalse(String message, boolean condition)           Asserts that a condition is false.
static void	assertNotEquals(double unexpected, double actual, double delta)           Asserts that two doubles are not equal to within a positive delta.
static void	assertNotEquals(float unexpected, float actual, float delta)           Asserts that two floats are not equal to within a positive delta.
static void	assertNotEquals(long unexpected, long actual)           Asserts that two longs are not equals.
static void	assertNotEquals(Object unexpected,Object actual)           Asserts that two objects are not equals.
static void	assertNotEquals(String message, double unexpected, double actual, double delta)           Asserts that two doubles are not equal to within a positive delta.
static void	assertNotEquals(String message, float unexpected, float actual, float delta)           Asserts that two floats are not equal to within a positive delta.
static void	assertNotEquals(String message, long unexpected, long actual)           Asserts that two longs are not equals.
static void	assertNotEquals(String message,Object unexpected,Object actual)           Asserts that two objects are not equals.
static void	assertNotNull(Object object)           Asserts that an object isn't null.
static void	assertNotNull(String message,Object object)           Asserts that an object isn't null.
static void	assertNotSame(Object unexpected,Object actual)           Asserts that two objects do not refer to the same object.
static void	assertNotSame(String message,Object unexpected,Object actual)           Asserts that two objects do not refer to the same object.
static void	assertNull(Object object)           Asserts that an object is null.
static void	assertNull(String message,Object object)           Asserts that an object is null.
static void	assertSame(Object expected,Object actual)           Asserts that two objects refer to the same object.
static void	assertSame(String message,Object expected,Object actual)           Asserts that two objects refer to the same object.
static <T> void	assertThat(String reason, T actual, Matcher<? super T> matcher)           Asserts that actual satisfies the condition specified by matcher.
static <T> void	assertThat(T actual,Matcher<? super T> matcher)           Asserts that actual satisfies the condition specified by matcher.
static void	assertTrue(boolean condition)           Asserts that a condition is true.
static void	assertTrue(String message, boolean condition)           Asserts that a condition is true.
static void	fail()           Fails a test with no message.
static void	fail(String message)

官网的断言示例如下：

import static org.junit.Assert.*;
import static org.hamcrest.CoreMatchers.*;  

import java.util.Arrays;
import org.hamcrest.core.CombinableMatcher;
import org.junit.Test;  

public class AssertTests {  

    @Test
    public void testAssertArrayEquals(){
        byte[] expected="trial".getBytes();
        byte[] actual="trial".getBytes();
        assertArrayEquals("failure-byte arrays not same",expected, actual);
    }  

    @Test
    public void testAssertEquals(){
        assertEquals("failure-strings are not equal","test","test");
    }  

    @Test
    public void testAssertFalse(){
        assertFalse("failure-should be false",false);
    }  

    @Test
    public void testAssertNotNull(){
        assertNotNull("should not be null",new Object());
    }  

    @Test
    public void testAssertNotSame(){
        assertNotSame("should not be same object",new Object(),new String("hello"));
    }  

    @Test
    public void testAssertNull(){
        assertNull("should be null",null);
    }  

    @Test
    public void testAssertSame(){
        Integer aNumber=Integer.valueOf(78);
        assertSame("should be the same",aNumber,aNumber);
    }  

      // JUnit Matchers assertThat
      @Test
      public void testAssertThatBothContainsString() {
        org.junit.Assert.assertThat("albumen", both(containsString("a")).and(containsString("b")));
      }  

      @Test
      public void testAssertThathasItemsContainsString() {
        org.junit.Assert.assertThat(Arrays.asList("one", "two", "three"), hasItems("one", "three"));
      }  

      @Test
      public void testAssertThatEveryItemContainsString() {
        org.junit.Assert.assertThat(Arrays.asList(new String[] { "fun", "ban", "net" }), everyItem(containsString("n")));
      }  

      // Core Hamcrest Matchers with assertThat
      @Test
      public void testAssertThatHamcrestCoreMatchers() {
        assertThat("good", allOf(equalTo("good"), startsWith("good")));
        assertThat("good", not(allOf(equalTo("bad"), equalTo("good"))));
        assertThat("good", anyOf(equalTo("bad"), equalTo("good")));
        assertThat(7, not(CombinableMatcher.<Integer> either(equalTo(3)).or(equalTo(4))));
        assertThat(new Object(), not(sameInstance(new Object())));
      }  

      @Test
      public void testAssertTrue() {
        assertTrue("failure - should be true", true);
      }
}