Java Equality not Behaving as Expected

Question

I'm currently working on an android application that uses a subclass of SimpleListAdapter to bind a ListActivity. The list that the Adapter binds to is of type List<HashMap<String, Object>> .

I have to following If statement in the list Adapter...

if (dataRow.get("HasLineup1").toString()) == "0")

This never evaluates to true for me, even when the Eclipse debugger says that dataRow.get("HasLineup1").toString() is equal to "0" in the inspection window.

The list of data is populated from an XML source with the line

Game.put("HasLineup1", attributes.getNamedItem("home_lineup").getNodeValue());

I managed to work around the issue by changing the If statement to

if (Integer.parseInt(dataRow.get("HasLineup1").toString()) == 0)

Can someone explain to me why the first If statement I used wasn't working? Java isn't my native language, but I can't for the life of me figure out what I'm doing wrong based on my .Net background.

Answer 1

The == operator compares the instance memory location for non-primitive types therefore this fails unless the two sides of the operand are the same instance. Whenever dealing with non-primitive types, use equals instead of == .

Answer 2

如果您使用的是string ，则应使用equals方法。

if (dataRow.get("HasLineup1").toString())equals("0"))

Answer 3

The equality operator == will do the following, based on what you're comparing:

• Two primitive types (eg int , char , ...): check if their values are the same.
• Two non-primitive types (class instances): check if the compared references point to the same object in memory.

In other words, for non-primitives == doesn't imply semantics. You'll want the equals method for that. It's defined on Object so it'll always be available. But its semantics are determine by how (and if) a specific class decides to override this method.

For String, it's a character-by-character comparison with the argument of the method.

Answer 4

Here is a link with references about the differences between == and equals. == compares references, while equals() compares the values.

http://leepoint.net/notes-java/data/expressions/22compareobjects.html

Answer 5

Use

if (dataRow.get("HasLineup1").toString()).equals("0"))

== (double equals) only compares the objects to see if they point to the same place in memory. the equals method actually compares the characters in the two strings to one another.

Answer 6

在使用“ ==”之前，必须在两个字符串上都使用方法“ equals”或调用.internal（）。