guava学习--Function、Predicate

Function用于同步转换。

Predicate用于过滤。

import java.util.Collection;

import java.util.Iterator;

import java.util.List;

import java.util.Map;

import com.google.common.base.Function;

import com.google.common.base.Functions;

import com.google.common.base.Objects;

import com.google.common.base.Predicate;

import com.google.common.collect.Collections2;

import com.google.common.collect.Iterators;

import com.google.common.collect.Lists;

import com.google.common.collect.Maps;

public class User {

private String username;

private String sex;

public User(String username, String sex) {

this.username = username;

this.sex = sex;

}

void setUsername(String username) {

this.username = username;

}

void setSex(String sex) {

this.sex = sex;

}

String getUsername() {

return username;

}

String getSex() {

return sex;

}

@Override

public String toString() {

return Objects.toStringHelper(this).omitNullValues().add("username", username).add("sex", sex).toString();

}

public static void main(String[] args) {

User u1 = new User("malone", "man");

User u2 = new User("lanlan", "woman");

List<User> list = Lists.newArrayList(u1, u2);

// Function 这里用于类型转换

Collection<String> result = Collections2.transform(list, new Function<User, String>() {

@Override

public String apply(User user) {

if (Objects.equal(user, null)) {

return "";

}

return user.toString();

}

});

for (String ss : result) {

System.out.println(ss);

}

// Predicate 这里是用于过滤

Collection<User> resultUser = Collections2.filter(list, new Predicate<User>() {

@Override

public boolean apply(User user) {

if (Objects.equal(user, null)) {

return false;

}

if (Objects.equal(user.getSex(), "woman")) {

return false;

}

return true;

}

});

for (User ss : resultUser) {

System.out.println(ss);

}

Function<User, String> f1 = new Function<User, String>() {

@Override

public String apply(User user) {

if (Objects.equal(user, null)) {

return "";

}

return user.getUsername();

}

};

// 多个工具类均可使用类函数式编程

Collection<String> result1 = Collections2.transform(list, f1);

for (String ss : result1) {

System.out.println("1:" + ss);

}

List<String> strList = Lists.transform(list, f1);

for (String ss : strList) {

System.out.println("2:" + ss);

}

Iterator<String> it = Iterators.transform(list.iterator(), f1);

while (it.hasNext()) {

System.out.println("3:" + it.next());

}

Function<String, String> f2 = new Function<String, String>() {

@Override

public String apply(String input) {

if (Objects.equal(input, "malone")) {

return "malone's function";

} else {

return "lanlan's function";

}

}

};

// 连接多个Function

// Warning：这里compose方法的参数位置不能颠倒，

//        * Function<A, C> compose(Function<B, C> g, Function<A, ? extends B> f)

//        * 传入Function<B,C>、Function<A, ? extends B>组合成Function<A, C>

Function<User, String> f3 = Functions.compose(f2, f1);

List<String> mylist = Lists.transform(list, f3);

System.out.println(mylist.get(0));

Map<String, String> map = Maps.newHashMap();

map.put("1", "2");

// Functions可以转换一下map，使用语义更直白的代码，并提供默认值支持

Function<String, String> lookup = Functions.forMap(map, "default value");

System.out.println(lookup.apply("1"));

}

}

Predicates:

Map<String, Girl> map = new HashMap<String, Girl>() {
{
put("love the age", new Girl(18, "not so nice"));
put("love the face", new Girl(16, "so nice"));
}
};
Function<String, Girl> function = Functions.forMap(map);
Predicate<Girl> predicate = new Predicate<Girl>() {
@Override
public boolean apply(Girl input) {
return input.getAge() >= 18;
}
};
Predicate<String> result = Predicates.compose(predicate, function);
return result;