Pass the Oracle Java SE 1z0-830 Questions and answers with ExamsMirror

In this code, there is a type mismatch between the DoubleStream and the Predicate.

DoubleStream: A sequence of primitive double values.

Predicate<Double>: A functional interface that operates on objects of type Double (the wrapper class), not on primitive double values.

The DoubleStream class provides a method anyMatch(DoublePredicate predicate), where DoublePredicate is a functional interface that operates on primitive double values. However, in the code, a Predicate is used instead of a DoublePredicate. This mismatch leads to a compilation error because anyMatch cannot accept a Predicate when working with a DoubleStream.

To correct this, the predicate should be defined as a DoublePredicate to match the primitive double type:

java

DoubleStream doubleStream = DoubleStream.of(3.3, 4, 5.25, 6.66);

DoublePredicate doublePredicate = d -> d < 5;

System.out.println(doubleStream.anyMatch(doublePredicate));

With this correction, the code will compile and print true because there are elements in the stream (e.g., 3.3 and 4.0) that are less than 5.

In this code, a list of integers integers is created containing the elements 0, 1, and 2. A stream is then created from this list, and the following operations are performed in sequence:

peek(System.out::print):

The peek method is an intermediate operation that allows performing an action on each element as it is encountered in the stream. In this case, System.out::print is used to print each element. However, since peek is intermediate, the printing occurs only when a terminal operation is executed.

limit(2):

The limit method is another intermediate operation that truncates the stream to contain no more than the specified number of elements. Here, it limits the stream to the first 2 elements.

forEach(i -> {}):

The forEach method is a terminal operation that performs the given action on each element of the stream. In this case, the action is an empty lambda expression (i -> {}), which does nothing for each element.

The sequence of operations can be visualized as follows:

Original Stream Elements: 0, 1, 2

After peek(System.out::print): Elements are printed as they are encountered.

After limit(2): Stream is truncated to 0, 1.

After forEach(i -> {}): No additional action; serves to trigger the processing.

Therefore, the output of the code is 01, corresponding to the first two elements of the list being printed due to the peek operation.

In this code, a Map named map is created using Map.of with the following key-value pairs:

"b": 1

"a": 3

"c": 2

The Map.of method returns an immutable map containing these mappings.

Next, a TreeMap named treeMap is instantiated by passing the map to its constructor:

java

TreeMap treeMap = new TreeMap<>(map);

The TreeMap constructor with a Map parameter creates a new tree map containing the same mappings as the given map, ordered according to the natural ordering of its keys. In Java, the natural ordering for String keys is lexicographical order.

Therefore, the TreeMap will store the entries in the following order:

"a": 3

"b": 1

"c": 2

When System.out.println(treeMap); is executed, it outputs the TreeMap in its natural order, resulting in:

r

{a=3, b=1, c=2}

Thus, the correct answer is option F: {a=3, b=1, c=2}.

In Java, array initialization can be performed in several ways, but certain syntaxes are invalid and will cause compilation errors. Let's analyze each declaration:

var array1 = new String[]{ "foo", "bar", "buz" };

This is a valid declaration. The var keyword allows the compiler to infer the type from the initializer. Here, new String[]{ "foo", "bar", "buz" } creates an anonymous array of String with three elements. The compiler infers array1 as String[]. This syntax is correct and compiles successfully.

var array2[] = { "foo", "bar", "buz" };

This declaration is invalid. While var can be used for type inference, appending [] after var is not allowed. The correct syntax would be either String[] array2 = { "foo", "bar", "buz" }; or var array2 = new String[]{ "foo", "bar", "buz" };. Therefore, this line will cause a compilation error.

var array3 = new String[3] { "foo", "bar", "buz" };

This declaration is invalid. In Java, when specifying the size of the array (new String[3]), you cannot simultaneously provide an initializer. The correct approach is either to provide the size without an initializer (new String[3]) or to provide the initializer without specifying the size (new String[]{ "foo", "bar", "buz" }). Therefore, this line will cause a compilation error.

var array4 = { "foo", "bar", "buz" };

This declaration is invalid. The array initializer { "foo", "bar", "buz" } can only be used in an array declaration when the type is explicitly provided. Since var relies on type inference and there's no explicit type provided here, this will cause a compilation error. The correct syntax would be String[] array4 = { "foo", "bar", "buz" };.

String array5[] = new String[]{ "foo", "bar", "buz" };

This is a valid declaration. Here, String array5[] declares array5 as an array of String. The initializer new String[]{ "foo", "bar", "buz" } creates an array with three elements. This syntax is correct and compiles successfully.

Therefore, the declarations that compile successfully are array1 and array5.

References:

Java SE 21 & JDK 21 - Local Variable Type Inference

Java SE 21 & JDK 21 - Arrays

java.io.Console is used for interactive input from the console.

Existing Methods in java.io.Console

reader() → Returns a Reader object.

readLine() → Reads a line of text from the console.

readLine(String fmt, Object... args) → Reads a formatted line.

readPassword() → Reads a password, returning a char[].

readPassword(String fmt, Object... args) → Reads a formatted password.

read() Does Not Exist

Consoledoes not have a read() method.

If character-by-character reading is required, use:

java

Console console = System.console();

Reader reader = console.reader();

int c = reader.read(); // Reads one character

read() is available inReader, butnot in Console.

Thus, the correct answer is:read() does not exist.

References:

Java SE 21 - Console API

Java SE 21 - Reader API