Why wait and notify is declared in Object class instead of Thread ?
Why multiple inheritance is not supported in Java ?
Reason is ambiguity around Diamond problem, consider a class A has foo() method and then B and C derived from A and has there own foo() implementation and now class D derive from B and C using multiple inheritance and if we refer just foo() compiler will not be able to decide which foo() it should invoke. This is also called Diamond problem because structure on this inheritance scenario is similar to 4 edge diamond.
Why String is immutable in Java?
String is Immutable in Java because String objects are cached in String pool. Since cached String literals are shared between multiple clients there is always a risk, where one client's action would affect all another client. For example, if one client changes the value of String "Test" to "TEST", all other clients will also see that value as explained in the first example. Since caching of String objects was important from performance reason this risk was avoided by making String class Immutable. At the same time, String was made final so that no one can compromise invariant of String class e.g. Immutability, Caching, hashcode calculation etc by extending and overriding behaviors. Another reason of why String class is immutable could die due to HashMap. Since Strings are very popular as HashMap key, it's important for them to be immutable so that they can retrieve the value object which was stored in HashMap. Since HashMap works in the principle of hashing, which requires same has value to function properly. Mutable String would produce two different hashcodes at the time of insertion and retrieval if contents of String was modified after insertion, potentially losing the value object in the map.
In Java in order to enter critical section of code, Threads needs lock and they wait for lock, they don't know which threads holds lock instead they just know the lock is hold by some thread and they should wait for lock instead of knowing which thread is inside the synchronized block and asking them to release lock. this analogy fits with wait and notify being on object class rather than thread in Java.
Why multiple inheritance is not supported in Java ?
Reason is ambiguity around Diamond problem, consider a class A has foo() method and then B and C derived from A and has there own foo() implementation and now class D derive from B and C using multiple inheritance and if we refer just foo() compiler will not be able to decide which foo() it should invoke. This is also called Diamond problem because structure on this inheritance scenario is similar to 4 edge diamond.
Why String is immutable in Java?
String is Immutable in Java because String objects are cached in String pool. Since cached String literals are shared between multiple clients there is always a risk, where one client's action would affect all another client. For example, if one client changes the value of String "Test" to "TEST", all other clients will also see that value as explained in the first example. Since caching of String objects was important from performance reason this risk was avoided by making String class Immutable. At the same time, String was made final so that no one can compromise invariant of String class e.g. Immutability, Caching, hashcode calculation etc by extending and overriding behaviors. Another reason of why String class is immutable could die due to HashMap. Since Strings are very popular as HashMap key, it's important for them to be immutable so that they can retrieve the value object which was stored in HashMap. Since HashMap works in the principle of hashing, which requires same has value to function properly. Mutable String would produce two different hashcodes at the time of insertion and retrieval if contents of String was modified after insertion, potentially losing the value object in the map.
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