安卓取消引用可能会产生“java”。lang.NullPointerException'

4 月，3 周 Questions & Answers 6786

以下代码段在Android Studio中生成一个lint警告

    Bundle extras = getIntent().getExtras();
    if (extras != null && extras.getString(REDIRECT_KEY) != null) {
        switch (extras.getString(REDIRECT_KEY)) { ...

extras.getString(REDIRECT_KEY)产生警告

Dereference of 'extras.getString(REDIRECT_KEY)' may produce 'java.lang.NullPointerException'

但我看不出会发生这种情况。这是不是lint检查中的一个bug，它以前根本不识别if中的null检查？还是我错过了什么

编辑：将代码更改为以下，是否删除了警告

if(getIntent() != null && getIntent().getStringExtra(REDIRECT_KEY) != null){
        switch (getIntent().getStringExtra(REDIRECT_KEY)){
            ...
        }
    }

但这只是因为这样，这种皮棉检查工作（至少我猜）。如果我在这张支票上显示更多信息，它会在某一点上显示

Variables, method parameters and return values marked as @Nullable or @NotNull are treated as nullable (or not-null, respectively) and used during the analysis to check nullability contracts

查看Bundle和Intent的源代码显示：

/**
 * Retrieve extended data from the intent.
 *
 * @param name The name of the desired item.
 *
 * @return the value of an item that previously added with putExtra()
 * or null if no String value was found.
 *
 * @see #putExtra(String, String)
 */
public String getStringExtra(String name) {
    return mExtras == null ? null : mExtras.getString(name);
}

和基束

/**
 * Returns the value associated with the given key, or null if
 * no mapping of the desired type exists for the given key or a null
 * value is explicitly associated with the key.
 *
 * @param key a String, or null
 * @return a String value, or null
 */
@Nullable
public String getString(@Nullable String key) {
    unparcel();
    final Object o = mMap.get(key);
    try {
        return (String) o;
    } catch (ClassCastException e) {
        typeWarning(key, o, "String", e);
        return null;
    }
}

如您所见，BaseBundle将其返回值设置为@Nullable，而Intent则不设置。因此，使用getStringExtra只会删除符号，而不会删除原因。我仍然猜测这是由于不充分的皮棉检查造成的，而不是我这边的错误编码。或者还有人看到可以抛出空指针的场景吗

class Argument { public final static int TOMAYTO = 0; public final static int TOMAHTO = 1; static void argue() { int say = TOMAYTO; while (true) { switch (say) { case TOMAYTO: say = TOMAHTO; break; case TOMAHTO: say = TOMAYTO; break; } } } }

0 iconst_0 // Push constant 0 (TOMAYTO) 1 istore_0 // Pop into local var 0: int say = TOMAYTO; 2 iload_0 // Push key for switch from local var 0 // Perform switch statement: switch (say) {... // Low case value is 0, high case value is 1 // Default branch offset will goto 2 3 tableswitch 0 to 1: default=2 0: 24 // case 0 (TOMAYTO): goto 24 1: 29 // case 1 (TOMAHTO): goto 29 // Note that the next instruction starts at address 24, // which means that the tableswitch took up 21 bytes 24 iconst_1 // Push constant 1 (TOMAHTO) 25 istore_0 // Pop into local var 0: say = TOMAHTO 26 goto 2 // Branch unconditionally to 2, top of while loop 29 iconst_0 // Push constant 1 (TOMAYTO) 30 istore_0 // Pop into local var 0: say = TOMAYTO 31 goto 2 // Branch unconditionally to 2, top of while loop

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安卓取消引用可能会产生“java”。lang.NullPointerException'

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