如何在沒有擴展Activity類的情況下從java類的assets文件夾中讀取文件？

Question

我想閱讀資產文件夾下的文件，然後我想從我的文件中獲取文本作爲Main_Activity類中的返回值。這是我的代碼。

Main.java

這是在主類中的方法。

ReadFile.java

這fileReaing（）方法是在ReadFile.java。

public String fileReading() throws IOException { 
    StringBuilder sb = new StringBuilder();  
    AssetManager am = getAssets(); 
    InputStream is = am.open("a.txt"); 
    BufferedReader br = new BufferedReader(new InputStreamReader(is)); 
    String line = null; 

    while ((line = br.readLine()) != null) { 
     sb.append(line + "\n"); 
    } 
    return sb.toString(); 
} 
} 

運行此代碼後，我發現以下錯誤。

Process: com.csh.readfile, PID: 4379 
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.csh.readfile/com.csh.readfile.Main}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.AssetManager android.content. 
Context.getAssets()' on a null object reference 

請幫我解決這個錯誤。我只是Android的初學者，並非常感謝所有人。

Answer 1

問題是你沒有通過context到fileReading()方法ReadFile.java，嘗試，如下面

ReadFile f= new ReadFile(); 
    try { 
     str = f.fileReading(getApplicationContext()); 
    } 

然後

public String fileReading(Context c) throws IOException { 
    StringBuilder sb = new StringBuilder();  
    AssetManager am = c.getAssets(); 
    InputStream is = am.open("a.txt"); 
    ..... 
    ..... 
} 

Answer 2

兩種可能性：

1）傳遞上下文你的方法。

public String fileReading(Context ctx) throws IOException { 
StringBuilder sb = new StringBuilder(); 
AssetManager am = ctx.getAssets(); 
InputStream is = am.open("a.txt"); 
BufferedReader br = new BufferedReader(new InputStreamReader(is)); 
String line = null; 
while ((line = br.readLine()) != null) { 
sb.append(line + "\n"); 
} 
return sb.toString(); 
} 

2）傳入上下​​文到你的構造

public class ReadFile{ 
private Context ctx; 

public ReadFile(Context ctx){ 
this.ctx = ctx: 
} 

public String fileReading() throws IOException { 
StringBuilder sb = new StringBuilder(); 
AssetManager am = ctx.getAssets(); 
InputStream is = am.open("a.txt"); 
BufferedReader br = new BufferedReader(new InputStreamReader(is)); 
String line = null; 
while ((line = br.readLine()) != null) { 
sb.append(line + "\n"); 
} 
return sb.toString(); 
} 
}