Guava Table 示例教程

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Guava Table 示例教程

本文带你学习Guava Table 接口及其实现。Guava Table 是类似表格结构的集合，包含行、列以及相应的单元格值。行和列作为有序的键。

概述

如果把Guava Table和java核心类中的集合进行比较，则为行map，且每行包含列map，其中有对应的单元格值。Table表示一个特定的map，有两个key以组合的形式引用单个值。类似场景一个map的map，举例：Map<UniversityName, Map<CoursesOffered, SeatAvailable>>。

依赖

maven依赖如下，读者可以查找对应版本：

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>21.0</version>
</dependency>

创建

有多种方式创建其实例：

• 使用HashBasedTable 类的create方法，其内部使用LinkedHashMap 类：

Table<String, String, Integer> universityCourseSeatTable 
  = HashBasedTable.create();

• 如果我们需要Table的行key和列key有序（按自然顺序或提供的比较器排序），可以使用TreeBasedTable的create方法，其内部使用TreeMap：

Table<String, String, Integer> universityCourseSeatTable
  = TreeBasedTable.create();

• 如何事先就确定行key和列可以，且table大小固定，可以使用ArrayTable的create方法：

List<String> universityRowTable 
  = Lists.newArrayList("Mumbai", "Harvard");
List<String> courseColumnTables 
  = Lists.newArrayList("Chemical", "IT", "Electrical");
Table<String, String, Integer> universityCourseSeatTable
  = ArrayTable.create(universityRowTable, courseColumnTables);

• 如果需要创建不可变的table实例，其内部数据不能修改，可以使用ImmutableTable 类，使用其builder：

Table<String, String, Integer> universityCourseSeatTable
  = ImmutableTable.<String, String, Integer> builder()
  .put("Mumbai", "Chemical", 120).build();

应用

下面通过简单示例说明如何应用Table：

取值

如果确定行key和列key，那么可以通过它们获取关联值：

@Test
public void givenTable_whenGet_returnsSuccessfully() {
    Table<String, String, Integer> universityCourseSeatTable 
      = HashBasedTable.create();
    universityCourseSeatTable.put("Mumbai", "Chemical", 120);
    universityCourseSeatTable.put("Mumbai", "IT", 60);
    universityCourseSeatTable.put("Harvard", "Electrical", 60);
    universityCourseSeatTable.put("Harvard", "IT", 120);
 
    int seatCount 
      = universityCourseSeatTable.get("Mumbai", "IT");
    Integer seatCountForNoEntry 
      = universityCourseSeatTable.get("Oxford", "IT");
 
    assertThat(seatCount).isEqualTo(60);
    assertThat(seatCountForNoEntry).isEqualTo(null);
}

检查项是否存在

可以通过下面几种方式检查table的项是否存在：

• 行key
• 列key
• 行key和列key
• 值

请看示例：

@Test
public void givenTable_whenContains_returnsSuccessfully() {
    Table<String, String, Integer> universityCourseSeatTable 
      = HashBasedTable.create();
    universityCourseSeatTable.put("Mumbai", "Chemical", 120);
    universityCourseSeatTable.put("Mumbai", "IT", 60);
    universityCourseSeatTable.put("Harvard", "Electrical", 60);
    universityCourseSeatTable.put("Harvard", "IT", 120);
 
    boolean entryIsPresent
      = universityCourseSeatTable.contains("Mumbai", "IT");
    boolean courseIsPresent 
      = universityCourseSeatTable.containsColumn("IT");
    boolean universityIsPresent 
      = universityCourseSeatTable.containsRow("Mumbai");
    boolean seatCountIsPresent 
      = universityCourseSeatTable.containsValue(60);
 
    assertThat(entryIsPresent).isEqualTo(true);
    assertThat(courseIsPresent).isEqualTo(true);
    assertThat(universityIsPresent).isEqualTo(true);
    assertThat(seatCountIsPresent).isEqualTo(true);
}

删除

可以通过行key和列key从table中删除项：

@Test
public void givenTable_whenRemove_returnsSuccessfully() {
    Table<String, String, Integer> universityCourseSeatTable
      = HashBasedTable.create();
    universityCourseSeatTable.put("Mumbai", "Chemical", 120);
    universityCourseSeatTable.put("Mumbai", "IT", 60);
 
    int seatCount 
      = universityCourseSeatTable.remove("Mumbai", "IT");
 
    assertThat(seatCount).isEqualTo(60);
    assertThat(universityCourseSeatTable.remove("Mumbai", "IT")).
      isEqualTo(null);
}

获取行key和单元格值的映射map

通过列key 获取行key和单元格值的映射map：

@Test
public void givenTable_whenColumn_returnsSuccessfully() {
    Table<String, String, Integer> universityCourseSeatTable 
      = HashBasedTable.create();
    universityCourseSeatTable.put("Mumbai", "Chemical", 120);
    universityCourseSeatTable.put("Mumbai", "IT", 60);
    universityCourseSeatTable.put("Harvard", "Electrical", 60);
    universityCourseSeatTable.put("Harvard", "IT", 120);
 
    Map<String, Integer> universitySeatMap 
      = universityCourseSeatTable.column("IT");
 
    assertThat(universitySeatMap).hasSize(2);
    assertThat(universitySeatMap.get("Mumbai")).isEqualTo(60);
    assertThat(universitySeatMap.get("Harvard")).isEqualTo(120);
}

获取table的map表示

通过使用columnMap方法，可以获取 Map<UniversityName, Map<CoursesOffered, SeatAvailable>> 形式的map结构：

@Test
public void givenTable_whenColumnMap_returnsSuccessfully() {
    Table<String, String, Integer> universityCourseSeatTable 
      = HashBasedTable.create();
    universityCourseSeatTable.put("Mumbai", "Chemical", 120);
    universityCourseSeatTable.put("Mumbai", "IT", 60);
    universityCourseSeatTable.put("Harvard", "Electrical", 60);
    universityCourseSeatTable.put("Harvard", "IT", 120);
 
    Map<String, Map<String, Integer>> courseKeyUniversitySeatMap 
      = universityCourseSeatTable.columnMap();
 
    assertThat(courseKeyUniversitySeatMap).hasSize(3);
    assertThat(courseKeyUniversitySeatMap.get("IT")).hasSize(2);
    assertThat(courseKeyUniversitySeatMap.get("Electrical")).hasSize(1);
    assertThat(courseKeyUniversitySeatMap.get("Chemical")).hasSize(1);
}

获取列key和单元格值的映射map

通过行key获取列key和单元格值的映射map：

@Test
public void givenTable_whenRow_returnsSuccessfully() {
    Table<String, String, Integer> universityCourseSeatTable 
      = HashBasedTable.create();
    universityCourseSeatTable.put("Mumbai", "Chemical", 120);
    universityCourseSeatTable.put("Mumbai", "IT", 60);
    universityCourseSeatTable.put("Harvard", "Electrical", 60);
    universityCourseSeatTable.put("Harvard", "IT", 120);
 
    Map<String, Integer> courseSeatMap 
      = universityCourseSeatTable.row("Mumbai");
 
    assertThat(courseSeatMap).hasSize(2);
    assertThat(courseSeatMap.get("IT")).isEqualTo(60);
    assertThat(courseSeatMap.get("Chemical")).isEqualTo(120);
}

获取唯一的行key

rowKeySet 方法可以获取所有行key，并自动去重：

@Test
public void givenTable_whenRowKeySet_returnsSuccessfully() {
    Table<String, String, Integer> universityCourseSeatTable
      = HashBasedTable.create();
    universityCourseSeatTable.put("Mumbai", "Chemical", 120);
    universityCourseSeatTable.put("Mumbai", "IT", 60);
    universityCourseSeatTable.put("Harvard", "Electrical", 60);
    universityCourseSeatTable.put("Harvard", "IT", 120);
 
    Set<String> universitySet = universityCourseSeatTable.rowKeySet();
 
    assertThat(universitySet).hasSize(2);
}

获取唯一的列key

columnKeySet 方法可以获取所有列key，并自动去重：

@Test
public void givenTable_whenColKeySet_returnsSuccessfully() {
    Table<String, String, Integer> universityCourseSeatTable
      = HashBasedTable.create();
    universityCourseSeatTable.put("Mumbai", "Chemical", 120);
    universityCourseSeatTable.put("Mumbai", "IT", 60);
    universityCourseSeatTable.put("Harvard", "Electrical", 60);
    universityCourseSeatTable.put("Harvard", "IT", 120);
 
    Set<String> courseSet = universityCourseSeatTable.columnKeySet();
 
    assertThat(courseSet).hasSize(3);
}

总结

本文我们介绍了Guava中Table类及其常用方法。table提供类似表格的结构，包括行、列以及关联值，在实际项目中非常有用。