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POReferences and variable passing. Explain the output of this simple java program?
 

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    <p><strong>Quick summary of what I need help with:</strong></p> <ul> <li>What's an immutable class?</li> <li>With Java's String class, why can you not 'point' one String variable at another (using C terminology) like you can with other classes? For example, <code>String x=(String)y;</code> copies y's value over to x. Why? <ul> <li>For example, with my own class ('Test'), <code>Test x=(Test)y;</code> does properly 'point' x at y. <br></li> </ul></li> <li>Does String's behaviour here have anything to do with its immutability?</li> </ul> <hr> <p>I have this Java program... can someone explain what's going on with the references?</p> <pre><code>public class Main { public static void foo(String a){ a="2"; } public static void main(String[] args) { String x="1"; foo(x); System.out.println("x="+x); } } </code></pre> <p>Output:</p> <pre><code>x=1 </code></pre> <p>Expected Output:</p> <pre><code>x=2 </code></pre> <p>What's going on? I thought that whatever I do to <code>a</code> within <code>foo</code> would also affect <code>x</code>, as <code>a</code> is just an alias for <code>x</code> - no?</p> <p>A similar issue here:</p> <pre><code>public class Main { public static void main(String[] args) { String x=new String("1"); String y=new String("2"); y=x; x="3"; System.out.println("x="+x); System.out.println("y="+y); } } </code></pre> <p>Output:</p> <pre><code>x=3 y=1 </code></pre> <p>Expected Output:</p> <pre><code>x=3 y=3 </code></pre> <p>Can someone explain this?</p> <hr> <p>Edit:</p> <p>Why do these examples produce the expected results and my above examples with String didn't?</p> <p>1:</p> <pre><code>public class Main { private static class Test{ public String val; public Test(String set){val=set;} public Test(){} } public static void foo(Test a){ a.val="2"; } public static void main(String[] args) { Test x=new Test("1"); foo(x); System.out.println("x="+x.val); } } </code></pre> <p>Output (and expected output):</p> <pre><code>x=2 </code></pre> <p>2:</p> <pre><code>public class Main { private static class Test{ public String val; public Test(String set){val=set;} public Test(){} } public static void main(String[] args) { Test x=new Test("1"); Test y=new Test("2"); x=y; x.val="3"; System.out.println("x="+x.val); System.out.println("y="+y.val); } } </code></pre> <p>Output (and Expected Output):</p> <pre><code>x=3 y=3 </code></pre>
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