Question

我需要将Map<String, String>类型的值传递给Android的AsyncTask。很遗憾，AsyncTask的回调doInBackground(Params... params)和execute(Params... params)方法接受了varargs。我们知道varargs会变成数组，我们无法创建泛型类型的数组。如何将参数传递给execute？

Answer 1

泛型主要用来警告你在编译时做坏事。要获得带有泛型的变量，可以使用如下所示的包装器：

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class VarArgsGeneric {

public static void main(String... args) {

    StringMapWrapper mw1 = createMap("1", "one");
    StringMapWrapper mw2 = createMap("2", "two");
    StringMapWrapper mw3 = createMap("3", "three");

    VarArg<StringMapWrapper> test = new VarArg<StringMapWrapper>();
    test.add(mw1, mw2, mw3);
    test.add((StringMapWrapper)null);
    StringMapWrapper[] mws = test.getParams(StringMapWrapper.class);

    System.out.println(Arrays.toString(mws));
}

public static StringMapWrapper createMap(String key, String value) {

    StringMapWrapper mwrapped = new StringMapWrapper(new HashMap<String, String>());
    mwrapped.getMap().put(key, value);
    return mwrapped;
}

static class VarArg<Param> {

    private final List<Param> allParams = new ArrayList<Param>();

    // Heap pollution: http://stackoverflow.com/q/12462079/3080094
    @SafeVarargs
    public final void add(Param... params) {

        for (Param p : params) {
            allParams.add(p);
        }
    }

    @SuppressWarnings("unchecked")
    public <T> T[] getParams(Class<T> type) {

        // Copied from http://stackoverflow.com/a/530289/3080094
        final T[] pa = (T[]) Array.newInstance(type, allParams.size());
        for (int i = 0; i < pa.length; i ++) {
            pa[i] = type.cast(allParams.get(i));
        }
        return pa;
    }
}

static class StringMapWrapper {

    private final Map<String, String> m;

    public StringMapWrapper(Map<String, String> m) {
        this.m = m;
    }

    public Map<String, String> getMap() {
        return m;
    }

    @Override
    public String toString() {
        return m.toString();
    }
}
}

哪个打印：
[{1=one}, {2=two}, {3=three}, null]

将泛型类型参数传递给syncTask

1 个答案: