将双嵌套for循环重写为Java 8流 [英] Rewrite double nested for loop as a Java 8 stream

问题描述

我有以下Java方法:

I have the following Java method:

public List<GrantedAuthority> toAuthorities(Set<Role> roles) {
    List<GrantedAuthority> authorities = new ArrayList<>();

    if (null != roles) {
        for (Role role : roles) {
            for (Permission permission : role.getPermissions()) {
                authorities.add(new SimpleGrantedAuthority("ROLE_" + permission.getLabel()));
            }
        }
    }

    return authorities;
}

我正在尝试使用Java 8流重写它.迄今为止我最大的尝试:

I'm trying to rewrite it using Java 8 streams. My best attempt thus far:

public List<GrantedAuthority> toAuthorities(Set<Role> roles) {
    List<GrantedAuthority> authorities = new ArrayList<>();

    if (null != roles) {
        roles.stream().filter(role -> ???).collect(Collectors.toList());
    }

    return authorities;
}

但是我对流过滤器中的内容(替换为???)感到迷茫……有什么想法吗?

But I'm at a loss as to what I put in the stream filter (substituting ???)...any ideas?

推荐答案

您可以使用flatMap和map instaead作为:

You can do it using flatMap and map instaead as :

if (null != roles) {
    authorities = roles.stream()
         .flatMap(role -> role.getPermissions().stream()) // Stream<Permission>
         .map(permission -> 
                 new SimpleGrantedAuthority("ROLE_" + permission.getLabel())) // Stream<SimpleGrantedAuthority>
         .collect(Collectors.toList());
}

在for循环代码中，您不会基于条件过滤/进行任何迭代，也不会在整个列表中进行迭代，因此在这里不需要filter.

In the for loop code, you are not filtering out/in any iteration based on a condition and iterating throughout the lists, hence you don't require a filter here.

使用上述方法，您的完整方法可以写成:

And using the above your complete method could be written as :

public List<GrantedAuthority> toAuthorities(Set<Role> roles) {
    return roles == null ? new ArrayList<>() : roles.stream()
            .flatMap(role -> role.getPermissions().stream())
            .map(permission -> new SimpleGrantedAuthority("ROLE_" + permission.getLabel()))
            .collect(Collectors.toList());
}

或者作为由shmosel建议，使用方法引用，可以将其转换为:

Or as suggested by shmosel, with method references this could be transformed as :

return roles == null ? new ArrayList<>() : roles.stream()
        .map(Role::getPermissions)
        .flatMap(Collection::stream)
        .map(Permission::getLabel)
        .map("ROLE_"::concat)
        .map(SimpleGrantedAuthority::new)
        .collect(Collectors.toList());

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