Java传递方法参考作为其他方法的参数 [英] Java pass method reference as parameter to other method

问题描述

我正在尝试将选定的A类获取"方法传递给B类中的方法.我已经签出解决方案

您的getX()方法可以看作是接受DataStore实例并返回浮点数的函数.

在Java 8中，您可以使用方法引用来表示它们:

    float[] aArray = getValuesAsArray(dataMap, DataStore::getA);
    float[] bArray = getValuesAsArray(dataMap, DataStore::getB);
    float[] cArray = getValuesAsArray(dataMap, DataStore::getC);

然后您的getValuesAsArray将接受Function<DataStore,Float>参数并执行功能:

private static float[] getValuesAsArray(Map<Integer, DataStore> dataMap, Function<DataStore,Float> func) {
    int i = 0;
    int nMap = dataMap.size();
    float[] fArray = new float[nMap];
    for (Map.Entry<Integer, DataStore> entry : dataMap.entrySet()) {
        DataStore ds = entry.getValue();
        fArray[i] = func.apply(ds);
        i++;
    }
    return fArray;
}

在不使用Java 8的情况下，您可以定义自己的接口，该接口包含一个接受DataStore实例并返回float的方法.然后，您不必使用Java 8方法引用，而必须将调用getX()方法之一的该接口的实现(可以使用实现该接口的匿名类实例)传递给getValuesAsArray方法.

例如:

public interface ValueGetter
{
    public float get (DataStore source);
}

float[] aArray = getValuesAsArray(dataMap, new ValueGetter() {public float get (DataStore source) {return source.getA();}});
float[] bArray = getValuesAsArray(dataMap, new ValueGetter() {public float get (DataStore source) {return source.getB();}});
float[] cArray = getValuesAsArray(dataMap, new ValueGetter() {public float get (DataStore source) {return source.getC();}});

还有

private static float[] getValuesAsArray(Map<Integer, DataStore> dataMap, ValueGetter func) {
    int i = 0;
    int nMap = dataMap.size();
    float[] fArray = new float[nMap];
    for (Map.Entry<Integer, DataStore> entry : dataMap.entrySet()) {
        DataStore ds = entry.getValue();
        fArray[i] = func.get(ds);
        i++;
    }
    return fArray;
}

I am trying to pass a selected "get"-method of class A to a method in class B. I have checked out Java Pass Method as Parameter, but I was not able to adopt the interface approach to my problem in a reasonable way. I would prefer to not use java 8 (lambdas) and if possible avoid reflection as well. My feeling is, that I am looking at my problem the wrong way. Here is the specific simplified example of what I am trying to accomplish:

I have a class containing some fields and get-methods:

public class DataStore {
    private float a;
    private float b;
    private float c;

    public float getA() {
        return a;
    }

    public float getB() {
        return b;
    }

    public float getC() {
        return c;
    }

}

Next I have my main class instantiating DataStore as Values of a Map and then accessing specific fields of DataStore like:

public class App {

    public static void main(String[] args) {
        // declare TreeMap using DataStore class as value
        Map<Integer, DataStore> dataMap = new TreeMap<Integer, DataStore>();

        // populate Map with example data
        dataMap.put(2,  new DataStore(1f,2f,3f));
        dataMap.put(10, new DataStore(3f,4f,5f));
        dataMap.put(4,  new DataStore(6f,7f,8f));

        // work with specific fields in DataStore, e.g. assign to array
        float[] aArray = getValuesAsArray(dataMap, DataStore.getA());
        float[] bArray = getValuesAsArray(dataMap, DataStore.getB());
        float[] cArray = getValuesAsArray(dataMap, DataStore.getC());
    }

    /**
     * Assign specific field of DataStore from Map to Array
     * @param dataMap
     * @param getVar - reference for specified getter method
     * @return 
     */
    private static float[] getValuesAsArray(Map<Integer, DataStore> dataMap, MethodReference getVar()) {
        int i = 0;
        int nMap = dataMap.size();
        float[] fArray = new float[nMap];
        for (Map.Entry<Integer, DataStore> entry : dataMap.entrySet()) {
            DataStore ds = entry.getValue();
            fArray[i] = ds.getVar();
            i++;
        }
        return fArray;
    }
}

Clearly this wont work, as I have to figure out how to pass my selected get method into getValuesAsArray(). Somehow, I guess, my approach may be wrong. So I am open for suggestions.

解决方案

Your getX() methods can be seen as a Function that accepts a DataStore instance and returns a float.

In Java 8 you can represent them with method references :

    float[] aArray = getValuesAsArray(dataMap, DataStore::getA);
    float[] bArray = getValuesAsArray(dataMap, DataStore::getB);
    float[] cArray = getValuesAsArray(dataMap, DataStore::getC);

Then your getValuesAsArray will accept a Function<DataStore,Float> parameter and execute the function :

private static float[] getValuesAsArray(Map<Integer, DataStore> dataMap, Function<DataStore,Float> func) {
    int i = 0;
    int nMap = dataMap.size();
    float[] fArray = new float[nMap];
    for (Map.Entry<Integer, DataStore> entry : dataMap.entrySet()) {
        DataStore ds = entry.getValue();
        fArray[i] = func.apply(ds);
        i++;
    }
    return fArray;
}

Without using Java 8, you can define your own interface that contains a method that accepts a DataStore instance and returns a float. Then, instead of using Java 8 method references, you would have to pass to your getValuesAsArray method an implementation of that interface (you could use an anonymous class instance implementing the interface) which calls one of the getX() methods.

For example :

public interface ValueGetter
{
    public float get (DataStore source);
}

float[] aArray = getValuesAsArray(dataMap, new ValueGetter() {public float get (DataStore source) {return source.getA();}});
float[] bArray = getValuesAsArray(dataMap, new ValueGetter() {public float get (DataStore source) {return source.getB();}});
float[] cArray = getValuesAsArray(dataMap, new ValueGetter() {public float get (DataStore source) {return source.getC();}});

And

private static float[] getValuesAsArray(Map<Integer, DataStore> dataMap, ValueGetter func) {
    int i = 0;
    int nMap = dataMap.size();
    float[] fArray = new float[nMap];
    for (Map.Entry<Integer, DataStore> entry : dataMap.entrySet()) {
        DataStore ds = entry.getValue();
        fArray[i] = func.get(ds);
        i++;
    }
    return fArray;
}

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