Implement Formattable interface in Java API References on HarmonyOS
0Here's how to Implement Formattable interface in Java API References on HarmonyOS.
i. Flags and length constraints refer to optional parameters and limits that can be defined on some methods, classes or interfaces in Java API references. Here are some examples:
Flags:
- Bitwise flags that can be set or combined using bit operators to enable or disable certain predefined behaviors or options. For example, the "File.createNewFile()" method takes an int flags parameter that can use flag constants like DO_NOT_CREATE_DIRECTORY.
- Enum flags that allow passing in enum values to enable options for a method. For example, the Pattern.compile() method takes a int flags parameter that allows passing enum bits like Pattern.CASE_INSENSITIVE.
Length Constraints:
- Maximum length values for strings/arrays/collections passed into a method or stored in a class. For example, Java class fields may define a MAX_LENGTH constant for a string.
- Minimum length values that require a certain size for passed in data. For example, key lengths in encryption APIs.
- Length-based overrides, where methods may have overloaded versions that take different lengths, like short vs int vs long arrays.
- Bounds on generic types that constrain the acceptable types and lengths, like T extends Number or E specifying an upper bound wildcard with a length limit.
So in summary, flags allow options to be selectively enabled, while length constraints ensure data size limitations are adhered to. These parameters provide more control over how Java APIs and data structures behave.
ii. Multithreaded access refers to having multiple threads access the same method, class or resource in Java concurrently. The Java API documentation provides some guidance around this:
1. Synchronization:
- Methods documented with the "synchronized" keyword can only be accessed by one thread at a time. This prevents race conditions.
2. Volatile variables:
- Fields marked "volatile" ensure updates are propagated across threads. Useful for flags, lifecycles etc.
3. Atomic classes:
- Wrappers like AtomicInteger use advanced techniques for thread-safe incremental access.
4. Concurrent collections:
- Special collections like ConcurrentHashMap support scalable concurrent access.
5. Reentrant locks:
- Allow manual locking using concurrency constructs like ReentrantLock for advanced cases.
6. Thread safety notes:
- Documentation calls out thread safety, if a class is not thread safe, or needs external synchronization.
So in summary, Java APIs provide various language facilities and custom data structures to allow safe, high performance concurrent access across multiple threads. Checking documentation for thread safety is important for writing robust multi-threaded Java code.
iii. A NullPointerException in Java occurs when you try to access or manipulate a reference that points to null rather than an actual object.
Some common examples that can cause a NullPointerException as noted in Java API documentation:
1. Uninitialized object references:
MyObject obj;
obj.doSomething(); // NullPointerException
2. Incorrect assumptions about primitives:
Integer num = null; // Compilation error
3. Accessing methods on null references returned from methods:
if(obj.getChild() != null) {
obj.getChild().method(); // Can still NPE if getChild() returns null
}
4. Using null instead of empty collections:
List list = null;
list.add("A"); // NullPointerException
5. Chaining method calls on objects that could be null:
class.getA().getB().doSomething(); // Any link in chain could NPE
To prevent NullPointerExceptions, check for null values before usage, use optionals, utilize defensive coding practices, and handle NPE gracefully.
The Java documentation highlights these common pitfalls that can lead to NullPointerExceptions during access of references pointing to null.
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