Java 何时需要显式类型参数？

鉴于：

import com.google.common.collect.ImmutableMap;
import java.util.function.Function;
import java.util.stream.Collector;
import java.util.stream.Stream;

public class Testcase
{
    public static <T, K, V> MapCollectorBuilder<T, K, V>
        toImmutableMap(Function<? super T, ? extends K> keyMapper,
            Function<? super T, ? extends V> valueMapper)
    {
        return null;
    }

    public static final class MapCollectorBuilder<T, K, V>
    {
        public Collector<T, ?, ImmutableMap<K, V>> build()
        {
            return null;
        }
    }

    public static <T, K, V> Collector<T, ?, ImmutableMap<K, V>> toImmutableMap2(
        Function<? super T, ? extends K> keyMapper,
        Function<? super T, ? extends V> valueMapper)
    {
        return null;
    }

    public void main(String[] args)
    {
        Function<String, String> keyMapper = i -> i;
        Function<String, Integer> valueMapper = Integer::valueOf;

        ImmutableMap<String, Integer> map1 = Stream.of("1", "2", "3")
            .collect(Testcase.toImmutableMap(keyMapper, valueMapper).build());

        ImmutableMap<String, Integer> map2 = Stream.of("1", "2", "3")
            .collect(Testcase.toImmutableMap(i -> i, Integer::valueOf).build());

        ImmutableMap<String, Integer> map3 = Stream.of("1", "2", "3")
            .collect(Testcase.toImmutableMap2(i -> i, Integer::valueOf));
    }
}

语句涉及和编译正常，但失败：map1map3map2

Testcase.java:[41,57] incompatible types: cannot infer type-variable(s) T,K,V
    (argument mismatch; invalid method reference
      no suitable method found for valueOf(java.lang.Object)
          method java.lang.Integer.valueOf(java.lang.String) is not applicable
            (argument mismatch; java.lang.Object cannot be converted to java.lang.String)
          method java.lang.Integer.valueOf(int) is not applicable
            (argument mismatch; java.lang.Object cannot be converted to int))

编译器错误可以通过提供显式类型参数来解决。<String, String, Integer>

1. Java 8 何时需要显式类型参数？也就是说，是否有一种已知的模式可以打破类型推断？
2. 是否可以更改以避免显式类型参数，而不会丢失使用生成器来配置收集器？toImmutableMap()MapCollectorBuilder

更新：

3. 为什么涉及的声明有效？它与涉及的声明有何不同？map3map2