[java] Group a list of objects by an attribute

you can use guava's Multimaps

@Canonical
class Persion {
     String name
     Integer age
}
List<Persion> list = [
   new Persion("qianzi", 100),
   new Persion("qianzi", 99),
   new Persion("zhijia", 99)
]
println Multimaps.index(list, { Persion p -> return p.name })

it print:

[qianzi:[com.ctcf.message.Persion(qianzi, 100),com.ctcf.message.Persion(qianzi, 88)],zhijia:[com.ctcf.message.Persion(zhijia, 99)]]

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