WHEN ARE TWO STRINGS EQUAL?
Strings are treated differently by Java developers than other first class objects. You can initialize a new String using new:
String string = new String("hello"); //not recommended
You can also use the following syntax to accomplish almost the same thing:
String string = "hello";
This tip addresses the question "when are two Strings equal?" Because they are objects, you can always compare the values of two Strings using the equals() method. If s1 and s2 are two Strings with the same value, then s1.equals(s2) is true. The tricky part comes when you try to use == to compare s1 and s2. In this tip you will see when s1 == s2 should return true.
To start with, create two objects of type Double, two primitives of type double, and two objects of type String. Use the s1 = "hello" syntax for initializing the Strings.
When your run Equals, you get the following results:
For Double objects both 7.2
object1 == object2 is false
object1.equals(object2) is true
For double primitives both 7.2
prim1 == prim2 is true
For Strings both 7.2
string1 == string2 is true
string1.equals(string2) is true
As you would expect, object1.equals(object2) is true because they have the same value but object1 == object2 is false because they are different objects. Also, prim1 == prim2 is true because they have the same value (note that you cannot use equals() to compare two primitives). For the objects string1 and string 2, string1.equals(string2) is true because their values are the same. Perhaps it is a surprise that string1 == string2 is true. The Java Language Specification explains that "Literal strings within the same class in the same package represent references to the same String object."
The Language Specification explains that "String literals - or, more generally, strings that are the values of constant expressions - are 'interned' so as to share unique instances using the method String.intern."
On the surface, the next example seems to highlight the differences between initializing a String as you would initialize an object, and initializing a String as you would initialize a primitive. It actually demonstrates the difference between obtaining a String by using a String literal directly, and by calling a String constructor.
Run NewEquals, and you will see these results:
For new Strings s1 and s2
s1 == s2 is false
s1.equals(s2) is true
For Strings s1 and s3
s1 == s3 is false
s1.equals(s3) is true
All three Strings are created with the same value, so equals() returns true for both comparisons. However, s1 and s2 are different objects. Despite being constructed with the same value, they are as different as the two Double objects were in the first example. This is why s1 == s2 is false. Similarly, s1 == s3 is also false.
You probably wouldn't be surprised by this result if it were not for the first example (Equals) where the two Strings returned true when compared with the == operator. It seems that these objects have the behavior expected of objects. The Equals example showed you that there is a pool for Strings. Multiple String variables can refer to the same String object. If they are set to equal string literals, they are guaranteed to do so.
The Language Specification guarantees that calling new will create a new object. That object has never been passed to String.intern. However, the value of a String literal is a String that has been passed String.intern, so it is different. The Language Specification does say that you can force a String into the common String pool using the intern() method, as shown in the following example:
Here are the results of running NewInternString:
For new Strings s1 and s2
s1 == s2 is false
s1.equals(s2) is true
For Strings s1 and s3
s1 == s3 is true
s1.equals(s3) is true
You can see that again the values are being reported as equal. That's because the equals() method returns true in both cases. After you force s1 into the constants pool you find that s1 == s3 is true. However, s2 is still not in the constants pool, so s1 == s2 is false.
In this tip you saw that the only safe way to test that Strings have the same value is with the equals() method. If you work with Strings that have the same value, then it might be worth interning them into the constants pool. That way you can make a quicker check using the == operator. In any case, you can see that you have to be careful before deciding to use == to check equivalence.
For more information on String comparisons see section 9.2 "String Comparisons" in The Java Programming Language Third Edition by Arnold, Gosling, and Holmes.
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A compile-time error occurs if it is impossible to convert the type of either operand to the type of the other by a casting conversion (�5.5).
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The detailed rules for compile-time correctness checking of a casting conversion of a value of compile-time reference type S (source) to a compile-time reference type T (target) are as follows:
If S is a class type:
If T is a class type, then S and T must be related classes-that is, S and T must be the same class, or S a subclass of T, or T a subclass of S; otherwise a compile-time error occurs.
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3.18 Passing Primitive Data Values
When the actual parameter is a variable of a primitive data type, the value of the variable is copied to the formal parameter at method invocation. Since formal parameters are local to the method, any changes made to the formal parameter will not be reflected in the actual parameter after the call completes.
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3.19 Passing Object Reference Values
If an actual parameter is a reference to an object, then the reference value is passed. This means that both the actual parameter and the formal parameter are aliases to the object denoted by this reference value during the invocation of the method. In particular, this implies that changes made to the object via the formal parameter will be apparent after the call returns. The actual parameter expression must evaluate to an object reference before the reference value can be passed.
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Ready-to-run state
A thread starts life in the Ready-to-run state (see p. 369).
Running state
If a thread is in the Running state, it means that the thread is currently executing (see p. 369).
Dead state
Once in this state, the thread cannot ever run again (see p. 380).
Non-runnable states
A running thread can transit to one of the non-runnable states, depending on the circumstances. A thread remains in a non-runnable state until a special transition occurs. A thread does not go directly to the Running state from a non-runnable state, but transits first to the Ready-to-run state.
The non-runnable states can be characterized as follows:
1. Sleeping: The thread sleeps for a specified amount of time (see p. 370).
2. Blocked for I/O: The thread waits for a blocking operation to complete (see p. 380).
3. Blocked for join completion: The thread awaits completion of another thread (see p. 377).
4. Waiting for notification: The thread awaits notification from another thread (see p. 370).
5. Blocked for lock acquisition: The thread waits to acquire the lock of an object (see p. 359)