Java 中的 Consumer 与 Supplier 接口使用场景与示例

@FunctionalInterface
public interface Consumer<T> {
    void accept(T t);
}

List<String> names = Arrays.asList("Alice", "Bob", "Charlie");
// 传统遍历
for (String name : names) {
    System.out.println("Hello, " + name);
}
// Consumer 实现
names.forEach(name -> System.out.println("Hello, " + name));
// 方法引用简化
names.forEach(System.out::println);

// 定义消费逻辑：将字符串转为大写并打印
Consumer<String> upperCaseConsumer = str -> {
    String upper = str.toUpperCase();
    System.out.println("Processed: " + upper);
};
upperCaseConsumer.accept("java"); // 输出: Processed: JAVA

// 组合多个消费操作
Consumer<Integer> print = num -> System.out.println("Number: " + num);
Consumer<Integer> square = num -> System.out.println("Square: " + num * num);
// 先打印再计算平方
print.andThen(square).accept(5);
/* 输出:
Number: 5
Square: 25
*/

@FunctionalInterface
public interface Supplier<T> {
    T get();
}

// 避免无意义的对象创建
Supplier<Connection> connectionSupplier = () -> {
    System.out.println("Creating new database connection...");
    return DriverManager.getConnection("jdbc:mysql://localhost/db");
};
// 仅在需要时调用get()创建对象
if (needDatabaseAccess()) {
    Connection conn = connectionSupplier.get();
    // 使用连接...
}

// 传统方式：先计算再判断，可能浪费资源
String result = expensiveCalculation();
if (result == null) {
    result = "default value";
}
// Supplier 优化：仅在需要时计算默认值
String optimizedResult = Optional.ofNullable(expensiveCalculation())
        .orElseGet(() -> {
            System.out.println("Using default value...");
            return "default value";
        });

// 定义对象创建供给者
Supplier<Person> personSupplier = () -> new Person("Default Name", 18);
// 批量创建对象
List<Person> defaultPersons = IntStream.range(0, 10)
        .mapToObj(i -> personSupplier.get())
        .collect(Collectors.toList());

// 定义数据处理流程：先由Supplier获取数据，再由Consumer处理
public static <T> void processData(Supplier<T> dataSupplier, Consumer<T> dataConsumer) {
    try {
        T data = dataSupplier.get(); // 惰性获取数据
        dataConsumer.accept(data);   // 处理数据
    } catch (Exception e) {
        System.err.println("Processing error: " + e.getMessage());
    }
}
// 使用示例
processData(
    () -> {
        System.out.println("Fetching data from database...");
        return jdbcTemplate.queryForObject("SQL", ResultSetExtractor);
    },
    data -> {
        System.out.println("Processing data: " + data);
        // 复杂数据处理逻辑
    }
);

// 使用Function转换数据，Consumer消费结果
List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
numbers.stream()
    .map(num -> num * 2)        // Function<Integer, Integer>
    .forEach(System.out::println); // Consumer<Integer>