相关疑难解决方法(0)

我是一名新的Android程序员,最近,我的很多项目都收到了这个错误:

07-31 23:45:19.592: ERROR/AndroidRuntime(716): java.lang.RuntimeException: Unable to start     

activity ComponentInfo{com.amrit.musifind/com.amrit.musifind.Main}: 

java.lang.NullPointerException

任何人都可以帮我解决这个问题吗？

这是整个logcat堆栈跟踪:

07-31 23:45:18.512: INFO/ActivityManager(63): Starting activity: Intent {     
act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10200000   
cmp=com.amrit.musifind/.Main }
07-31 23:45:18.682: INFO/ActivityManager(63): Start proc com.amrit.musifind for activity   
com.amrit.musifind/.Main: pid=716 uid=10025 gids={3003}
07-31 23:45:18.962: INFO/jdwp(716): received file descriptor 20 from ADB
07-31 23:45:19.102: DEBUG/ddm-heap(716): Got feature list request
07-31 23:45:19.572: DEBUG/AndroidRuntime(716): Shutting down VM
07-31 23:45:19.572: WARN/dalvikvm(716): threadid=3: thread exiting with uncaught   
exception (group=0x4001aa28)
07-31 23:45:19.572: ERROR/AndroidRuntime(716): Uncaught handler: thread main exiting due 
to uncaught …

我无法理解源代码Arrays.copyOf.

public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
    T[] copy = ((Object)newType == (Object)Object[].class)
        ? (T[]) new Object[newLength]
        : (T[]) Array.newInstance(newType.getComponentType(), newLength);
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

1. 这行检查是什么？

   (Object)newType == (Object)Object[].class
   

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2. (T[]) new Object[newLength]和之间有什么区别(T[]) Array.newInstance(newType.getComponentType(), newLength)？为什么Array.newInstance两种情况都不够好？

3. 以下行编译,但在运行时崩溃(如预期的那样).我什么时候应该使用这种方法？

   Integer[] nums = Arrays.copyOf(new String[]{"a", "b"}, 2, Integer[].class) 
   

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这个问题是关于"为什么autoboxing在Java中使一些调用变得模棱两可？"

但通过阅读答案,有许多参考铸造,我不确定我完全理解差异.

有人可以提供简单的解释吗？

public class Demo 
{
    public static void main(String[] args) 
    {
       int a=10,b=20;
       short c = (a<b)?a:b;
       System.out.println(c);
    }
}

这是我的程序,我得到以下错误,为什么我没有得到

"Demo.java:6: error: incompatible types: possible lossy conversion from int to short
short c = (a<b)?a:b;
1 error" 

我用变量声明写"final",它工作正常.但为什么会这样呢？

public class Demo 
{
    public static void main(String[] args) 
    {
        final int a=10,b=20;
        short c = (a<b)?a:b;
        System.out.println(c);
    }
}

我的程序有一个方法,可以获取特定行的计数并将其转换为BigInteger:

private BigInteger getSameSatPremise(ServiceAgreement sa) {
BigInteger count = BigInteger.ZERO;

    StringBuffer queryHql = new StringBuffer();
    queryHql.append("from Table t1");
    Query query = createQuery(queryHql.toString());
    query.addResult("count", "count(distinct t1.column1)");
    if (query.listSize() > 0) {
        count = (BigInteger) query.firstRow();
    }
    return count;
}

当查询结果为0时,转换工作正常.但是当查询结果为2时,我得到如下所示的转换错误.

Caused by: java.lang.ClassCastException: java.lang.Long cannot be cast to java.math.BigInteger

谁能帮忙.