Question

我正在设计一种加密货币跟踪器，该基本URL不适用于我需要的特定数据。我读到我的错误很可能是由于API URL调用不正确引起的。

错误

java.lang.NullPointerException: Attempt to invoke interface method 
'java.lang.Object java.util.List.get(int)' on a null object reference

基本网址

https://rest.coinapi.io/

http请求

GET /v1/symbols?filter_symbol_id={filter_symbol_id}

我的代码

String BASE_URL = "https://rest.coinapi.io/";

@GET("v1/symbols")
Call<List<Coin>> getCoin(@Query("filter_symbol_id") String filter_symbol_id);
}

代码调用API

 @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate (savedInstanceState);
    setContentView (R.layout.activity_main);

    final ListView listView = findViewById (R.id.listView);

    Retrofit retrofit = new Retrofit.Builder ()
            .baseUrl (Api.BASE_URL)
            .addConverterFactory (GsonConverterFactory.create ())
            .build ();

    Api api = retrofit.create (Api.class);

    Call<List<Coin>> call = api.getCoin ("filter_symbol_id");

    call.enqueue (new Callback<List<Coin>> () {
        @Override
        public void onResponse(Call<List<Coin>> call, 
Response<List<Coin>> response) {

            List<Coin> coin = response.body ();

            String[] coinName = new String[10];

            for (int i = 0; i < 10; i++) {


                coinName[i] = coin.get (i).getsymbol_id ();


                listView.setAdapter (
                        new ArrayAdapter<String> (
                                getApplicationContext (),
                                android.R.layout.simple_list_item_1,
                                coinName

                        )
                );

            }
        }


        @Override
        public void onFailure(Call<List<Coin>> call, Throwable t) {
            Toast.makeText (getApplicationContext (), t.getMessage (), 
Toast.LENGTH_SHORT).show ();
        }
        });

    }

}

Coin.java

package com.dev20tx.android.cryptohero;

public class Coin {

private String symbol_id;


public Coin(String symbol_id) {
    this.symbol_id = symbol_id;


}

public String getsymbol_id() {
    return symbol_id;
}
}

我发现了许多有用的stackoverflow帖子，这些帖子可以帮助我获取其他数据，但这使我感到困惑。有人可以指出我正确的方向吗？

已解决的问题：我使用了另一个URL“ v1 / assets”，该URL修复了NullPointerException，更新了我在Java文件中的代码，并且可以正常工作。出于某些原因，coinapi.io

目前无法使用API v1 / symbols

Answer 1

您丢失了api_key

@GET("v1/symbols")
Call<List<Coin>> getCoin(@Query("filter_symbol_id") String filter_symbol_id, @Query("apikey") String apiKey);

有关更多信息，请访问https://docs.coinapi.io/#authorization

Answer 2

该错误必须在您的Coin类中。因此，请发布Coin.class。我认为NullPointerException是因为GSON无法序列化Coin对象，可能是@SerializedName("")丢失了。否则显示Coin.class代码。（请记住也缺少apikey参数，因此响应将为401 Unauthorized，但不会引起任何NullPointerException。）

package com.dev20tx.android.cryptohero;

public class Coin {

@SerializedName("symbol_id") //replace with your response field.
private String symbol_id;


public Coin(String symbol_id) {
    this.symbol_id = symbol_id;
}

public String getsymbol_id() {
    return symbol_id;
}
}

网址查询改造2

2 个答案: