Guava中针对集合的 filter和过滤功能

Posted 2020-11-18 winner-0715

在guava库中，自带了过滤器(filter)的功能，可以用来对collection 进行过滤，先看例子： 

import com.google.common.base.Predicates;
import com.google.common.collect.Iterables;
import com.google.common.collect.Lists;

import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");
        Iterable<String> result = Iterables.filter(names, Predicates.containsPattern("a"));
        // [Jane, Adam]
        System.out.println(result);
    }
}

在这个例子中，给出一个list，过滤出含有字母a的元素 
此外，可以使用Collections2.filter() 去进行过滤 

import com.google.common.base.Predicates;
import com.google.common.collect.Collections2;
import com.google.common.collect.Lists;

import java.util.Collection;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");
        Collection<String> result = Collections2.filter(names, Predicates.containsPattern("a"));
        // [Jane, Adam]
        System.out.println(result);
    }
}

再来看下predicates判断语言， 
com.google.common.base. Predicate : 根据输入值得到 true 或者 false 
拿Collections2中有2个函数式编程的接口：filter , transform ,例如 ：在Collection<Integer>中过滤大于某数的内容：

import com.google.common.base.Predicate;
import com.google.common.collect.Collections2;
import com.google.common.collect.Lists;

import java.util.Collection;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<Integer> collections = Lists.newArrayList(1, 2, 3, 4);
        Collection<Integer> filter = Collections2.filter(
                collections, new Predicate<Integer>() {
                    @Override
                    public boolean apply(Integer input) {
                        return input >= 3;
                    }
        });
        // [3, 4]
        System.out.println(filter);
    }
}

把Lis<Integer>中的Integer类型转换为String , 并添加test作为后缀字符

import com.google.common.base.Function;
import com.google.common.collect.Lists;

import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<Integer> list = Lists.newArrayList(1, 2, 3, 4);
        List<String> transform = Lists.transform(list, new Function<Integer, String>() {
            @Override
            public String apply(Integer input) {
                return input + "_test";
            }
        });
        // [1_test, 2_test, 3_test, 4_test]
        System.out.println(transform);
    }
}

需要说明的是每次调用返回都是新的对象，同时操作过程不是线程安全的。 

将多个prdicate进行组合 

import com.google.common.base.Predicates;
import com.google.common.collect.Collections2;
import com.google.common.collect.Lists;

import java.util.Collection;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");
        Collection<String> result = Collections2.filter(names,
                Predicates.or(Predicates.containsPattern("J"),
                Predicates.not(Predicates.containsPattern("a"))));
        // [John, Jane, Tom]
        System.out.println(result);
    }
}

上面的例子中找出包含J字母或不包含a的元素; 
再看下如何将集合中的空元素删除： 

import com.google.common.base.Predicates;
import com.google.common.collect.Collections2;
import com.google.common.collect.Lists;

import java.util.Collection;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<String> names = Lists.newArrayList("John", null, "Jane", null, "Adam", "Tom");
        Collection<String> result = Collections2.filter(names, Predicates.notNull());
        // [John, Jane, Adam, Tom]
        System.out.println(result);
    }
}

检查一个collection中的所有元素是否符合某个条件： 

import com.google.common.base.Predicates;
import com.google.common.collect.Iterables;
import com.google.common.collect.Lists;

import java.util.List;

public class Test {
    public static void main(String[] args) {
        List<String> names = Lists.newArrayList("John", "Jane", "Adam", "Tom");

        boolean result = Iterables.all(names, Predicates.containsPattern("n|m"));
        // true
        System.out.println(result);
        result = Iterables.all(names, Predicates.containsPattern("a"));
        // false
        System.out.println(result);
    }
}