我怎样才能在 groovy 中做一个有条件的 collectEntries

Question

是否可以像 collect 这样有条件的 collectEntries ？

Answer 1

[ a:1, b:2, c:3, d:4 ].findAll { it.value > 2 }

应该做

Answer 2

It's not as succinct as Tim Yates's answer using findAll ; but just for the record, you can use collectEntries to do this:

[ a:1, b:2, c:3, d:4 ].collectEntries { 
    it.value > 2 ? [(it.key) : it.value] : [:] }

which evaluates to

[c:3, d:4]

Using "${it.key}" as done in this answer is an error, the key will end up being an instance of the GStringImpl class, not a String. The answer itself looks ok in the REPL but if you check what class it is, you can see it is wrong:

groovy:000> m = [ a:1, b:2, c:3, d:4 ]
===> [a:1, b:2, c:3, d:4]
groovy:000> m.collectEntries { ["${it.key}" : it.value ] }
===> [a:1, b:2, c:3, d:4]
groovy:000> _.keySet().each { println(it.class) }
class org.codehaus.groovy.runtime.GStringImpl
class org.codehaus.groovy.runtime.GStringImpl
class org.codehaus.groovy.runtime.GStringImpl
class org.codehaus.groovy.runtime.GStringImpl
===> [a, b, c, d]

Code trying to equate GroovyStrings to normal strings will evaluate to false even when the strings look identical, resulting in a hard-to-figure-out bug.

Answer 3

This should work:

[a:1, b:2, c:3, d:4].collectEntries {
    if (it.value > 2)
        ["${it.key}": it.value]
}

Answer 4

it works now after adding else. thanks

[a:1, b:2, c:3, d:4].collectEntries {
    if (it.value > 2){
        ["${it.key}": it.value]
    }else{
      [:]
    }
}